the rate of production of gas in C is equal to the sum of the rates of production in c, and c, together.

Further, whether C1, C2, and C be all sulphuric acid voltameters, or all copper voltameters, or all silver volta

Fig. 13.-Voltameters C2 and C, in Parallel with One Another, but in Series with Voltameter C1.

meters, or, indeed, all voltameters of the same character, it will be found that, no matter what be the shapes or sizes of the different voltameters, and no matter what be the areas of the platinum, copper, or silver plates immersed in the

respective liquids, or the distances apart of the plates, the amount of chemical action produced in a given time in C1 is almost exactly equal to the sum of the amounts of chemical action produced in C, and c, together. The plates, in any one of the voltameters, C1, may be large or small, near together or far apart may be, in fact, moved about while the chemical action is going on. The current may be strong and the chemical action take place rapidly, or it may be weak and the action proceed slowly, and it may be varied while the action is progressing; but the same general result still remains true. Measure the amount of chemical action that has taken place in c2 and in C, add the two together, and it will be found to be practically equal to the action that has taken place in C1 in the same time.

Now, when a river divides in consequence of the existence of an island in mid-stream, we know that the number of gallons of water flowing per minute on each side of the island must together equal the total number of gallons per minute flowing in the main stream, simply because the water which does not go past one side of the island must go past the other; and similarly, if we are to look upon a current of electricity in the same way as a current of water, we must expect that, when it divides into two parts, the sum of these parts must always be equal to the whole, whether the current which divides is a large one or a small one. The experiment just described shows that, if we say that a current is directly proportional to the rate at which chemical action is produced in a voltameter, this statement will always be true, whatever be the current in the main circuit; but it will not generally be true if we take any of the other effects occurring in the instruments indicated in Fig. 7 (page 11) as a direct measure of a current. Thus, in Fig. 13, if C1, C2, C3 represent galvanoscopes, such as A in Fig. 7, the deflection of the first will not generally be equal to the sum of the deflections of the other two; and even if this were the case for one current in

the main circuit, it would not be the case for any other. Nor will any simple relation be found to connect the deflection of the first instrument with those of the other two, unless elaborate precautions be taken in the construction of the apparatus.

6. Definition of the Unit Current; Ampere. We may, therefore, define the strength of a current as being proportional to the amount of chemical decomposition it can produce in a given time; and an unvarying current which, when passed through a solution of nitrate of silver in water, deposits silver at the rate of 0.001118 of a gramme per second, is taken as a current of one

ampere.

The metal deposited by the current does not adhere well to the plate of a voltameter or "electrolytic cell," if the action proceeds too rapidly; also errors will arise in the estimation of a current by the electrolytic method, unless certain precautions be carefully attended to. Thus, when measuring a current of about one ampere

with a silver voltameter, it is advisable to adopt the following arrangement: The cathode," sometimes spelt "kathode," or plate on which the silver is deposited, should take the form of a light bowl K (Fig.

14), not less than 10 centimetres * in diameter, and from 4 to 5 centimetres in depth, and made of platinum, so that it may be easily cleaned with nitric acid. The "anode," or plate from which the silver is electrically removed, should be a disc of pure silver, a, of about 30 square centimetres in area, and from 2 to 3 millimetres thick.

Riveted to the anode is a strip of pure silver which is braized to a brass strip s, and by means of the metal clamp c and nut N the anode is held so that its edge is equidistant all round from the rim of the cathode, and its upper surface just below the level of the liquid; this may conveniently consist of a neutral solution of pure silver nitrate, containing about 15 parts by weight of the salt to 85 parts of distilled water.

Electric contact is made between the wire w1 and the bowl by means of three metal pins p, on which the latter rests; and the wire w, is electrically joined to the anode disc by the strip s being held fast in the metal clamp c, to which the wire w, is attached.

In addition to the surface of the anode plate being turned into silver nitrate by the passage of the current, there is a tendency for small bits of silver to become detached and to fall into the bowl, thus making its weight too great. To prevent this, the anode should be wrapped round with pure filter paper, secured at the back with sealing-wax.

When making an observation, the current should be allowed to pass for about half an hour, and be maintained as constant as possible.

The preceding is based on the Reports issued in 1891, 1892, and 1894 by the Committee † appointed to advise

*One metre is 39.370 inches, therefore 10 centimetres correspond with a little less than 4 inches. One square metre is 1,550 square inches, therefore 30 square centimetres is a little less than 4 square inches.

+ This Committee consisted of Sir Courtenay Boyle, Mr. Hopwood and Major Cardew representing the Board of Trade; Mr. Preece and the late Mr. Graves representing the Postal Telegraph Department;

the Board of Trade on Electrical Standards, and the following, extracted from the "Order in Council" made by Her Majesty on the 23rd of August, 1894, is their description of the

"METHOD OF MAKING A MEASUREMENT.

"The platinum bowl is washed with nitric acid and distilled water, dried by heat, and then left to cool in a desiccator. When thoroughly dry, it is weighed carefully.

"It is nearly filled with the solution, and connected to the rest of the circuit by being placed on a clean copper support, to which a binding screw is attached. This copper support must be

insulated.

"The anode is then immersed in the solution, so as to be well covered by it and supported in that position; the connections to the rest of the circuit are made.

"Contact is made at the key, noting the time of contact. The current is allowed to pass for not less than half an hour, and the time at which contact is broken is observed. Care must be taken that the clock is keeping correct time during this interval.

"The solution is now removed from the bowl, and the deposit is washed with distilled water, and left to soak for at least six hours. It is then rinsed successively with distilled water and absolute alcohol, and dried in a hot-air bath at a temperature of about 160°C. After cooling in a desiccator it is weighed again. The gain in weight is the silver deposited.

"To find the current in amperes, this weight, expressed in grammes, must be divided by the number of seconds during which the current has been passed, and by 001118.

"The result will be the time-average of the current, if during the interval the current has varied.

"In determining by this method the constant of an instrument, the current should be kept as nearly constant as possible, and the readings of the instrument taken at frequent observed intervals of time. These observations give a curve from which the reading corresponding to the mean current (time-average of the current) can be found. The current, as calculated by the voltameter, corresponds to this reading."

Fig. 15 shows a desiccator, such as is referred to in the extract from the Board of Trade Reports. The

Lord Kelvin and Lord Rayleigh the Royal Society; Prof. Carey Foster and Mr. Glazebrook the British Association; and Dr. J. Hopkinson and the Author the Institution of Electrical Engineers.