comes to its final state after the wire has been violently disturbed, as in the last experiment, as also the fact of there being a permanent as well as a temporary effect, seem to render this hypothesis more probable than that the current is actually produced by a change in the molecular state of the wire.

The phenomena obtained in the last experiment will be rendered more clear by a diagram.

The intervals between taking off and putting on the weight were approximately equal. These are therefore represented by equal distances along the axis OT; and the strength of the current is set off along the axis OC. Starting from the point O, at the beginning of the experiment, with the weight attached to the wire as it had been left, then at any time the broken line represents the permanent change in the current produced by taking off or putting on the weight;

......

represents the temporary change;

represents the resultant strength of the current, being the sum of these two components.

The curve A represents the change in the current when the weight is left permanently attached; and the curve A' represents the change in the current when the weight is permanently removed.

At this stage in the experiments the method of measuring the current-strength in terms of a standard current was adopted. The battery used as a standard was a "sawdust" Daniell's (Menotti's) cell; and the strength of the current was approximately that produced by 1 volt through 10,000,000 ohms, or

10-8 C.G.S. unit. The measurements of the Tables are given in millionths of a C.G.S. unit.

Experiment 6.-A similar steel wire. -16°. Initial deflection barely perceptible S.U. Weight of 30 lbs. left on about 40 hours.

It was observed that at the time of making the experiment the weight was making small oscillations; and this appeared to be the cause of the deflections making small oscillations about a mean value. At the end of an hour and a half the oscillations of the weight and also of the deflection had ceased, the latter remaining steady at 5, indicating a current ⚫007 U.S. The weight was now made to perform vibrations of small amplitude, upon which the oscillations of the deflection were greatly increased both in number and amplitude, and the mean deflection was at the same time somewhat increased. If the vibration of the weight be suddenly stopped, it is some little time before a decrease is perceived in the oscillations of the deflection.

After setting the weight in gentle vibration, the effect in causing oscillations in the deflection was observable in less than a minute. If the vibrations of the weight are kept up for some time, the mean deflection is increased up to a certain limit, as before described. If the vibrations of the weight are increased in amplitude, the oscillations of the deflection become much more irregular, and the limits of variation become greater.

Experiment 7.-A similar wire. O=12. Initial current ⚫0014 U.S. A weight of 3 lbs. was now attached; and at the end of two minutes there was a current 0052 S.U., falling at the end of an hour and a half to 0034 U.S. The weight was then increased by 3 lbs. at a time and the deflections taken immediately, with the results given in the accompanying Table:

In the experiments after this the weights are given in terms of measures of shot, each of which weighed about 7480 grains.

Experiment 8.-A similar wire. 33 measures left on for about 40 hours. -12°. Deflections read immediately after removal of weight. Direction of current S.U. There was no initial deflection.

Experiment 9.-A similar wire.

tion. Deflections read immediately after application of weight.

-12°. No initial deflec

Experiment 10.-A steel wire 47 millim, diameter. -12°. Initial deflection 4 U.S. On attaching the empty can for containing the shot to the end of the lever the deflection increased to 20 U.S., falling to 14.5. The strain was gradually increased by pouring shot into the can until the wire broke. The deflection changed very little until the wire began to stretch, when the deflection fell very rapidly, passed through zero, and went up to about 40 S.U.

The more rapid the stretching the stronger is the current produced. When the strain was slightly lessened, so as to stop the stretching, the deflection fell very quickly to 20 S.U. On removing the strain the deflection fell rapidly, passed through

zero, and went up to 2 or 3 U.S., making irregular vibrations. The weight was replaced and additional shot poured in very slowly. The deflection almost instantaneously changed to about 2 S.U., which increased slightly until the wire broke. Experiment 11.-A copper wire 24 millim. diameter. =15°. Initial deflection 1 S.U.

The direction of the current was S.U. Several small weights were added to the can; but the deflection remained steady at 1.5. In copper wire, no fall in the deflection was observed when the weight was left suspended for some time.

The Physical Laboratory, University College, London.

XLVIII. On Galvanic Currents occasioned by Differences of Concentration-Inferences from the Mechanical Theory of Heat. By Professor HELMHOLTZ*.

WE will regard as the electro-chemical equivalent of an

sponding electrode, in the unit of time, by the chosen unit of

current.

The transport-number n, referred to the cation (Hittorf's), gives, as with Wiedemann, that fraction of the equivalent of the cation in question which is carried by the unit of current, during the unit of time, through each cross section of the current's path in the solution, to the cathode. On the other hand, the quantity (1-n) of the anion goes in the opposite direction, * Translated from the Monatsbericht der königlich preussischen Akademie der Wissenschaften zu Berlin, Nov. 1877, pp. 713-726.

by which (1-n) of the cation at the cathode becomes freewhich, combined with the amount n of cation brought to this side, gives the quantity 1 set free at the cathode. In like manner the quantity n of the cation is conveyed away from the other side, by which n of the anion is set free. To this is added (1-n) of the anion brought over. Now, when the cation is a metal which can deposit itself on the electrode, (1-n) of the metal disappears there from the solution, and (1-n) of the salt-forming acid is conveyed away; consequently from there (1-n) of the salt is removed. On the other side the liberated anion combines with the metal of the electrode; and therefore 1 equivalent of new metal here enters the solution, while n of the metal is carried away and (1-n) of the anion is brought over. This gives here an increase of the quantity of the salt by (1-n) of the equivalent for the unit of time and unit of current. If the metal of the electrode is the same as that which is contained in the solution, the total result of the electrolysis is the same as if one equivalent of metal were carried from the anode to the cathode, and (1-n) equivalent of salt in the solution from the cathode to the anode.

If, then, the salt-solution is more concentrated at the cathode than at the anode, the difference of concentration is equalized by the transfer. Therewith the liquid approaches the state of equilibrium to which the forces of attraction between the water and salt tend even in the processes of diffusion, namely the state of uniform distribution of the salt. Thus the chemical forces acting in this direction will also in turn assist the electric current acting in their direction.

That the work of the chemical forces which herewith comes in acts in this case as an electromotive force according to the same laws as other electrolytic chemical processes, can be deduced from the mechanical theory of heat.

A reversible process without changes of temperature, such as is required for the application of Carnot's law, we can institute in the following manner:

(1) We let the quantity E of positive electricity slowly enter the anode in a constant current, and in return take away the quantity + E from the cathode; or, what leads to the same result, we admit +E into the anode, and, inversely, discharge -E at the cathode. If P and P, are the values of the electrostatic-potential function for the two electrodes, then is

k

E{P-P}

the work which must be done in order to bring about this through-current. If the duration of the current is equal to t, the current-intensity according to electrostatic measure is given by the equation Jt=E.