When the series field winding is opposed to the shunt and weakens the field with increase of current, we have a differential compound motor. This is of use where it is necessary to keep the speed more closely constant with varying loads, and where extreme overloads are not found, as in textile mills. This type has one drawback, however. At heavy starting loads the shunt field is so overcome by the heavy armature current, in the differential series field, that the motor may start in the wrong direction. It is therefore customary to shortcircuit the series winding until normal speed is reached. The speed and torque characteristics

by changing the field current by rheostat. Commercially motors of this type are built for a four to one speed range. Some motors are provided for greater speed range by shifting the armature axially with respect to the field.

To limit the heavy current during the starting of motors, resistances are connected in series with the motor during starting. The actual change of connections is made by a controller which incorporates protective devices. These are described under ELECTRICAL MACHINERY.

ALTERNATING-CURRENT MoTORS: Synchronous Motors.-If alternator is running at

an

FIG. 17.-Three Horsepower, 230 Volt, Constant-Speed, DirectCurrent Motor

of these types of motors are shown in Fig. 16. (Torque curves are shown with broken lines.) With modern motors the regulation is such that differential compounding is both unnecessary and undesirable. In fact, many shunt motors have a few cumulative series turns to give stability.

In order to improve commutation, nearly all direct-current motors are built with interpoles. This permits more efficient use of materials in the machine and reduces its physical size for a given rating. Motors designed for heavy duty, as steel mill drive, where the load varies rapidly, are provided with a distributed winding in the pole face connected in series with the armature to neutralize the armature reaction and the consequent danger of flashing over the commutator.

Shunt interpole motors may be built for wide speed adjustment

normal speed, and generating E.M.F. in synchronism and about in phase with the circuit E.M.F., the difference between the two causes a current to flow through the alternator. When the alternator is being driven, the difference in E.M.F. is such as to cause a current which, reacting on the magnetic field, retards the armature. However, if the machine is not driven, the armature starts to lag slightly and the difference in E.M.F. is such as to cause a current which, reacting on the magnetic field, produces a torque to maintain motion at synchronous speed, and the machine becomes a synchronous motor.

When the field of the synchronous motor is increased so that the generated E.M.F. is greater than the circuit E.M.F., the difference causes a current which leads the voltage of the circuit and does not appreciably affect the torque produced in the motor. If, on the other

hand, the field is decreased, the resulting current lags the voltage of the circuit. It is this property of the synchronous motor to change the phase relation of its current, rather than its constant speed, that makes it most useful in practice. By field change the phase relation of its current or power factor may be changed, and hence the power factor of the circuit is changed. On transmission systems synchronous machines are used solely for modifying the phase of the circuit, usually without any mechanical load. (See ELECTRICAL MACHINERY.)

The above brief description applies to either single or polyphase motors. However, as with alternators, the torque and power in the single-phase machine pulsate, while in the polyphase (usually two- or three-phase) machine the torque and power are practically constant. Except in very special cases, singlephase synchronous motors are not used.

Synchronous motors have no inherent starting torque, and although occasionally they are provided with auxiliary motors to bring them up to synchronous speed, polyphase motors are usually provided with an amortisseur or damper winding placed in the pole faces, which is utilized as in the induction motor during starting. This damper winding also furnishes greater stability during normal operation. Part voltage is impressed on the armature producing the revolving field, as in the induction motor; this rotating field reacts with the damper winding to produce the required torque. The field circuit is shorted through a suitable resistance, so that its current (by induction) aids the damper winding in producing torque. When nearly up to synchronous speed, the field circuit is connected to its D.C. supply, causing the machine to lock in synchronism. Now full voltage is impressed on the windings, and the motor is ready to take its load. Synchronous Converters. has already been shown how a direct-current generator might have slip rings, so that it would also give alternating currents. Such a machine may be run as a synchronous motor, taking energy from a proper alternatingcurrent circuit and delivering direct current from the commutator. The machine is then a rotary converter. The ratio of effective (alternating) E.M.F. to the direct-current voltage is about 0.71 for single-phase machines, 0.50 for a two-phase, and 0.61 for a three-phase.

It

Ordinarily, to change the directcurrent voltage supply, as is

needed sometimes for electricrailway work at times of very heavy load, it is necessary to raise or lower the alternatingcurrent voltage by changing the transformer connections by induction regulators or by raising the generator voltage in the distant power house.

Large converters frequently have a synchronous booster as a part of the converter. This is a small alternator in series with the converter, by means of which a small adjustable voltage (usually 15 per cent. maximum) is added to (or subtracted from) the normal circuit voltage between the converter slip rings and the converter proper. This permits a corresponding change in the direct-current voltage at the commutator.

Induction Motors, Polyphase.The two essential parts of the induction motor are (1) the stator or stationary part and (2) the rotor or revolving portion. The stator is provided with a winding essentially the same as the armature of the revolving field alternator; this winding is connected to the supply circuit. The rotor is either of the squirrel cage type (without any direct electrical connection) see Fig. 18 or the phase wound type with slip rings for connection to a resistor.

In the polyphase alternator the load current produces a magnetic field (by armature reaction) which is practically constant in magnitude, but rotating at the same speed as the field structure or at synchronous

speed. With similar currents forced through the similar windings on the stator of the induc

FIG. 18

tion motor, a similar rotating magnetic field is produced. In the rotating field so produced there is introduced a laminated cylindrical iron core mounted on a shaft so as to be capable of rotation, and through a series of holes around the periphery of this core are threaded bars of copper, which at either end are welded or soldered to a connecting ring of copper. Such a form of winding is known as a squirrelcage or short circuited winding, and is shown in Fig. 18.

If the rotor stands still, voltages are induced in the copper bars, since the flux and conductors move relatively. Since the bars are short-circuited, currents

flow nearly proportional to the induced E.M.F.'s. We have shown that when current flowed in such a conductor situated in a magnetic field, a torque was produced, so that the rotor tends to revolve, and in a direction to prevent the relative motion of flux and conductor; that is, the rotor follows the magnetic field. It is evident that the motor cannot run synchronously with the magnetic field, for then the flux and conductors would be relatively fixed, so that no current would flow, and no torque be

of circuit by a lever actuating a sliding ring on the rotating shaft. A certain limited range of speed control with induction motors is obtained by arranging the field windings so that they can be connected to form different numbers of poles. The greater the number of poles the slower the speed. These changes are often made by contact pieces on a drum which can be rotated past a set of contact fingers.

Polyphase induction motors are the most rugged motors built, particularly the squirrel-cage

developed. Therefore, the rotor lags behind the speed of the magnetic field just enough to permit a current and to develop a torque equal to the mechanical resistance that must be overcome.

Although induction motors are self-starting, there is a large rush of current into the stator windings before the motor runs up to speed; and the magnetic field is much disturbed, owing to the excessive current produced in the rotor. As a result, the starting torque is poor. In most cases squirrel-cage motors are started with low voltages to reduce line disturbances. This further reduces the starting torque. In order to remedy these defects, coil-wound rotors (instead of short-circuited ones) are frequently used in the larger motors, the winding being arranged so as to allow of the introduction of starting resistances into the rotor circuits through slip rings. This has the double effect of reducing the starting current and increasing the starting torque. motor gains speed, the starting resistance is gradually reduced and ultimately short-circuited. In some cases the starting resistances are carried within the framework of the rotor in such a manner that after being started from rest with the aid of these resistances, they may be cut out

As the

type, which has no moving contacts. Because of this simple ruggedness, they are in common use for all purposes where constant-speed motors are desired. Machine tools, locomotives, and battleships are all successfully driven by induction motors. With high-resistance rotors, squirrel-cage motors, as well as the phase-wound rotor motors, are used for hoisting and elevator work. See ELECTRICAL MACHINERY.

Induction motors are built for various speeds and in sizes from a fraction of a horsepower up to 25,000 horsepower.

Single-Phase Motors. Series Type.-Reduced to their simplest form, such motors are the same as series direct-current motors. When current in both armature and field reverses simultaneously, the torque continues in the same direction. With a shunt motor placed on alternating current this simultaneous reversal does not occur, owing to the greater selfinduction of the field circuit. With the series motor it does happen, as the same current flows in both. The difficulties with series motors are (1) eddy losses in the iron field frame, due to alternating magnetic flux, and (2) heavy currents in the coils momentarily short-circuited by the brushes on the commutator

(see Figs. 3 and 4), with resultant vicious sparking. To overcome these troubles, the field magnets are built up of laminated iron, and small resistances are connected between the commutator segments and their respective armature coils.

The alternating magnetic flux sets up an E.M.F. in the armature and in the field windings which is 90° behind the current. These have the undesirable effect of giving a low-power factor, and of keeping the current and torque at starting close to that at maximum speed. Compensating windings are therefore inserted in slots in the pole faces, to set up in the armature E.M.F.'s which counteract those due to the reversals of current and flux, and permit the starting torque to be fixed as a desirable multiple of the full-speed torque. Then, and only then, does this motor become useful for railway work. These motors can operate on direct-current. The singlephase electric locomotives of the New York, New Haven, and Hartford Railroad so operate in entering the Grand Central Terminal of the New York Central and Hudson River Railroad in New York City.

The

Among the single-phase alternating-current commutator motors, the Thomson repulsion type stands out prominently. This has a stator giving a magnetic field like the single-phase series motor, and it has an armature like that of a direct-current motor. pair or pairs of brushes bearing on the commutator are shortcircuited and placed at an angle to the axis of the field. The magnetic flux through the armature, caused by the field, may be considered to have two components - one perpendicular to the brushes, and one parallel thereto. The latter component causes current in the armature coils, and the former component reacts, developing torque.

There are several other forms of repulsion motors. The Atkinson type has two field coils at right angles, and the short-circuited brushes lying in the axis of one of the field coils. One field then serves to generate currents in the short-circuited conductors, and the other furnishes the field necessary for torque. In the compensated repulsion motor, or the Latour - Winter - Eichberg, there are one stator coil and two sets of brushes. The magnetic effect is the same as in the Atkinson and Thomson types. The electrical actions are much complicated by the E.M.F. reactions set up by having one set of short-circuited windings, which serve as a field coil, cut the flux set up by armature current led in through the series brushes. The

effect is the beneficial one of decreasing the apparent reactance of the stator circuit in running, and of improving power factor and operation. The Deri repulsion motor has been developed to a commercial success in Europe. In its simplest form there are the the usual commutator, rotor, and stator. There are two fixed brushes diametrically opposite and in the axis of the stator coils. There are also two movable brushes which are always diametrically opposite. Each movable brush is connected to the nearest fixed brush, and the number of short-circuited conductors can be changed from none up to those

Thomson. Atkinson. Compnstd. Deri.

FIG. 20

com

included in an angle of 160°. The stator winding acts as a transformer, inducing current in the rotor conductors short-circuited. These rotor currents can be considered as having two ponents, one of which produces a magnetic flux, which gives torque with the currents flowing. The torque increases with increase of angle between short-circuited brushes. These four prominent types of repulsion are shown in diagrammatical arrangement in Fig. 20.

Single-phase Induction Motors. -The construction of singlephase induction motors is much like that of a two-phase induction motor witn the second phase not used. After the rotor is in motion, the rotor conductors cut the flux and generate an E.M.F. by dynamo action. This is combined with the E.M.F. generated in the rotor by pure transformer action, and the resultant currents give a magnetic flux which has a component in quadrature, mechanically and electrically, with the main rotor flux. This sets up a more or less perfect rotating field, so that, once started, the machine acts similar to a polyphase induction motor. Inherently there is no starting torque in the singlephase induction motor. Consequently some provision must be made to start the motor. The starting torque can be produced in very small motors by embedding heavy copper shading coils in the pole faces. The currents in these coils retard change in part of the main flux, and the persisting part of the field reacts on the rotor currents, producing torque enough to start. Splitphase motors are polyphase types in which the second- or third

phase field windings are used only for starting. By placing resistance, capacity, or inductance in those other field circuits, the currents in these other fields may be made to 'lag' behind the main field current enough to give a component which produces more or less of a revolving fieldenough for starting.

One very successful motor has the rotor conductors connected to a commutator, and it starts as a repulsion motor. After getting up to proper speed an automatic centrifugal clutch short-circuits the commutator and lifts the brushes, so that the motor continues to run as a single-phase induction type.

Another type of motor utilizes two windings, a normal repulsion winding with commutator and a squirrel-cage winding. The former has the greater effect at starting, producing a large starting torque; the squirrel-cage type has the greater effect under running conditions, giving a fairly constant speed characteristic.

Consult Fox' Principles of Electric Motors and Control (1924).

Dynamom'éter, any apparatus for measuring force or power, as, for example, the power developed by a steam engine or other motor. In practical engineering a common type is the brake, or absorption dynamometer, so-called because the energy is absorbed by frictional resistance. The entire energy so absorbed is converted into heat.

Fig. 1 shows a convenient form It of brake for small engines. consists of a rope encircling the fly-wheel, one end of the rope being

FIG. 1

attached to a spring-balance, and the other loaded with a suitable weight. In the figure, two parallel turns of the rope are shown passing round the wheel, but more may be used if desirable. Wood blocks are attached at intervals to keep the ropes in position. The direction of rotation is such as to tend to lift the weights, and, in working, the weights should be lifted a short

distance from the ground. This is easily arranged by raising or lowering the spring balance. A stop of some sort should be provided to prevent the weights being thrown over the wheel by any chance. If T1 be the weight (in lbs.) hung on the rope, T. the tension indicated on the balance, N the revolutions per minute, and D the diameter of the wheel (in ft.), measured to the centre of the rope, then brake horse-power .D.N.(T-T2).

33,000

It is not necessary that the rope should be applied round an exact circumference. In small engines a brake is sometimes used embracing only half the circumference. The above expression for the horsepower applies equally to this case. To remove the heat generated, the inside of the rim is sometimes cast channel-shaped, and a jet of water run into it, a scoop removing the heated water. Centrifugal force keeps the water in the rim. For large powers a steel band has been used, working on a wooden pulley, the band being kept cool by water circulating in a canvas jacket fastened to the back of the band.

Transmission Dynamometers, as the name implies, transmit the power to be measured without other loss than the friction of the apparatus, and this by careful construction may be made small.

Fig. 2 represents the Tatham dynamometer. P is the belt from the source of power driving the small pulley on the lower shaft. The motion is transmitted by means of a belt passing over the pulley v to the pulley c, and thus to the belt o connected to the machine or shaft to be driven. The pulleys K and M are loose pulleys supported on a lever pivoted at L. The motion of L is limited by stops.

When at rest, the belt s u will have an initial tension (T, say) which will be the same throughout its length, and there will be a downward pull of 2T at the centres of each of the pulleys K and м; and since the distances K L and L M are equal, there will be no tendency to rotate the lever. If now the machine be set in motion, power being transmitted from belt P to belt o, there will be a tension T1, say, in the tight side s of the belt, and a smaller tension T, in the slack side U. These two tensions will produce downward pulls of 2T,, and 2T, on the pulleys к and м respectively, and therefore there will be a turning moment on the lever of 2T1-X-2T2.X=2x(T1—T2). This is balanced by the sliding weight which will give a known turning moment, so that knowing x we obtain the value of T-T2, from which we get the horse-power transmitted, viz.

In another type of transmission dynamometer there are two pulleys, one being keyed to the shaft and the other riding loose on the shaft, but connected to the other pulley by springs, some arrangement being added to show the amount of relative motion of the two pulleys, from which the twisting moment exerted can be estimated. Their revolutions being known, the horse-power is easily calculated.

Traction Dynamometers.-For finding the force exerted in drawing

FIG. 2.

a vehicle such as a street-car or railway train, the pull is usually transmitted through a powerful spring balance, which indicates the actual force used in moving the vehicle. See Pullen's Experimental Engineering (1900); Carpenter's Experimental Engineering (1902).

Dyne, in dynamics, is the absolute unit of force in the C. G. S. system, which was introduced by the committee of the British Association in 1874, and defined as the force which, acting on a mass of one gram, will give it an acceleration of one centimetre per second. It is too small to be used as a convenient unit in practical life, the megadyne (= 1,000,000 dynes) being a little more than the weight of a kilogram. One pound avoirdupois is equal to 0.445 megadyne.

Dysentery, an acute or chronic inflammatory disease involving the intestines. It is peculiar to hot countries, but epidemic at times in temperate zones. According to its cause, it is divided into two forms, bacillary and amabic.

BACILLARY DYSENTERY is caused by many different types of bacilli. Infection is carried from the patient by clothes soiled with stools -containing the organism, and by water and food similarly contaminated. The disease, which usually has an acute course, prevails especially among people living in unsanitary surroundings, and is frequent in armies in the field. Summer diarrhoea in infants in the United

States is often due to infection by some variety of the dysentery bacillus.

Symptoms.-These are due to the absorption of the toxins of the bacilli in the body. The characteristic beginning is diarrhoea and colic. There is also painful straining during defecation. The matter discharged from the bowel soon becomes slimy, then blood-streaked; then only pure mucus and blood are passed; in severe cases the stools contain pus and shreds of mucous membrane. Later, if the attack is not stopped, casts of mucous membrane are passed, and the sloughing may include all tissues of the intestine, leading to perforation, followed by peritonitis. Gangrene may next attack the intestine; in this case, the matter voided becomes dark, with few or no casts, the solid matter taking the form of fine, dark powder. Usually, death occurs in severe cases in two or three weeks.

is se

Treatment.-Prevention cured by care in diet, and in the disposal of all excreta from any patient. During an epidemic the slightest attacks of diarrhoea should be most carefully attended to. Patients should be kept absolutely at rest and warmly covered. With regard to diet for adults, some advocate milk, arrowroot, soda-water, and brandy; while others say that experience is against milk in any form, and advocate beef-tea and bread and butter. At the onset of summer diarrhoea in infants, milk is to be stopped at once, and the patient given barley water or rice water. The food should not be given either hot or cold. For adults, ipecacuanha powder, in large doses, after castor oil and opium, is highly recommended by those who have seen much dysentery in India. Enemata of 1-per-cent. salt solution are recommended. Calomel or castor oil is often given early in the disease, and opium and bismuth later. Douching the intestine with methylene-blue solution is advised by some.

AMOEBIC DYSENTERY is characterized by a variable onset, irregular course, alternate diarrhoea and constipation, abdominal pain, and amabæ and sometimes mucus and blood in the stools. The disease was first recognized in the United States in 1890. It is prevalent in the Southern States, and occurs occasionally in most of the other States. It is chiefly found in young adult males. The amoeba is a unicellular protozoon parasite, and varies in diameter from 10 to 50μ. The principal source of infection is the water supply.

Symptoms.-The onset is usually indefinite: some lassitude or headache for several weeks-then diarrhoea and abdominal pain-then increasing frequency of stools, with mucus and blood. Relief may come

with proper treatment, or the condition may become worse, with fever, exhaustion, and death; or a chronic stage may develop, with anæmia, emaciation, and intermittent diarrhoea alternating with constipation. This condition may last several years, but even chronic conditions recover in some cases after a few months.

Complications. The most frequent is abscess of the liver, which occurs in about 20 per cent. of the cases, and may develop at any time in the course of the disease.

Diagnosis.-This is made by the discovery under the microscope of motile amabæ in the stools. The symptoms alone are insufficient for the diagnosis.

Treatment. When the disease is prevalent only sterilized drinking water should be used. In the acute stage rest, opium, and liquid diet are indicated. Curative treatment consists in high rectal irrigations of some antiseptic given once or twice daily. The antiseptic most used is a 1-to-1,000 solution of quinine, but silver nitrate solutions are also used.

Dysodil, in mineralogy, a laminated bituminous substance found in limestone near Syracuse in Sicily, green or gray in color, and often containing remains of fishes and of plants. It is a kind of natural bitumen.

Dyspepsia, or INDIGESTION, is a medical term which covers a very wide field. The symptoms of indigestion may be due to organic disease of the stomach or intestines, or to the improper performance of the digestive functions when the alimentary canal is not diseased. In the latter case, dyspepsia may be due to insufficient mastication, the result of bad teeth or too much haste in eating. The food is thus not properly broken up, in which case it is neither mixed with saliva nor in a condition to be easily reached by the gastric juice later. To drink

at meals is to encourage the swallowing of food without proper salivation, and dyspepsia is therefore sometimes relieved by stopping all drinking at meals, when each mouthful must be well mixed with saliva before it can be swallowed. In such cases a full quantity of fluid should be taken between meals.

When the food reaches the
VOL. IV.-Jan. '11.

stomach, it is thoroughly mixed with the gastric juice by a churning movement of the stomach walls, while the pylorus remains closed. If, however, there is insufficient gastric juice, or insufficient movement, a stomach indigestion results. These two deficiencies may be due to overeating, fatigue, or depressing emotions. As a result, there is undigested food, possibly in small quantities, which retards the passage of the digested food through the pyloric opening of the stomach into the duodenum. After this food has been a certain time in the stomach fermentation is set up, and water-brash, heartburn, flatulence, eructations, and even vomiting follow. Intestinal dyspepsia may be started by unhealthy conditions of liver or pancreas. See DIGESTION.

Among the commonest symptoms of chronic dyspepsia in adults are a bad taste in the mouth, particularly on waking, a feeling of oppression, even after a light meal, shortness of breath, eructations, heartburn, flatulence, distention of the abdomen, constipation often alternating with diarrhoea, headache, and dimness of vision. Perhaps the most distressing feature of simple chronic indigestion is that the dyspeptic's whole life is colored by his complaint.

Digestion is often much improved if each of the day's meals, though differing from the others, is itself simplified. Thus, it is useful to take the necessary carbohydrates apart from the proteids-making breakfast, for example, of toast, bread, porridge, and the like, and taking little or no farinaceous food at the meat-meal. Hot or cold water should be drunk freely either before rising in the morning or immediately after. Fluid is less likely to interfere with digestion if taken with breakfast than with any other meal; and coffee has a decided aperient action on many. Food that is eaten with a relish is, as a rule, wholesome, but usually the dyspeptic should avoid greasy foods and sweets. Dry and lean foods give most dyspeptics least trouble. New bread is a food which the dyspeptic should never eat. Tea, especially when not freshly made, or when strong, is a very common cause of indigestion. Bitter tonics

may be used to excite appetite by increasing the buccal and gastric secretions. Alkalies or acids are administered according to the effect desired. Taking sodium bicarbonate affords the commonest relief to slight indigestion. Carminatives, like peppermint, tend to disperse flatulence. Emetics give relief where acute dyspepsia is due to a recent meal, while, when the time for emetics has gone by, soothing remedies like bismuth may be used for the irritated stomach. Simple biliary defects are treated with cholagogues; and when the trouble has an intestinal origin, it is treated with a suitable aperient and purgative. Regular action of the bowels is important, not only to remove the indigestible residue of food, but to get rid of injurious products formed during digestion. While suitable drugs are often temporarily of the greatest assistance, they must rarely be habitually used, and should be taken as seldom and as sparingly as possible. Massage and electricity are often very useful aids, particularly in constipation. The dyspeptic should rest for at least a quarter of an hour after each meal, and should take the principal meal in the middle of the day, if there is then time to eat it quietly. Daily exercise is very important.

Dytiscus, a genus of large and ferocious water-beetles. Both larva and adult are aquatic, the latter diving and swimming with much rapidity. Both are actively carnivorous, and are dangerous occupants for an aquarium, for even small fish may fall victims to their strong jaws. The adult is about one and a quarter inches long. Swimming is effected by the powerful posterior legs, which are compressed and hairy, forming oars. The wing-covers fit perfectly, so that an air-tight space lies between them and the body. This space is filled with air at the surface, and from it the air passes gradually into the air-tubes (trachea), which open beneath the elytra. When the air is exhausted, the beetle rises to the surface in order to obtain a fresh supply. Wings are present, and are used to convey the insect from one pool to another.

Dziggetal. See KULAN.
Dzungaria. See ZUNGARIA.