XIII. The silver voltameter

Abstract

It has been known for several years that the measurement of electric quantity by the electro-deposition of silver is liable to inaccracies which appear to be dependent on the condition of the anode, kathode, and electrolyte of the voltameter (or coulo-meter) employed. In 1884 Lord Rayleigh and Mrs. Sidgwick found that a small quantity of silver acetate added to a solution of silver nitrate or of silver chlorate apparently increased the mass of silver deposited per ampere-second, and in 1895 Rodger and Watson showed that the silver voltameter was liable to give results varying by as much as 1 part in 1000 when the same solution of silver nitrate was repeatedly used. The latter effect was thought to be due to the formation of a complex silver salt, and in 1899 Richards, Collins, and Heimrod practically confirmed this view. A new form of silver voltameter was suggested by the latter observers, and this has been frequently used for observations of precision. In 1898 Kahle made a very large number of measurements, using platinum and silver bowls as kathodes and silver nitrate solutions, treated in various manners and from many sources, as electrolytes. He found that the deposit of silver per colomb was greater on a silver surface than on one of platinum; that it increased with the continued use of a solution, and that the nature of the deposit also varied with the solution employed. In 1892 Schuster and Crossley discovered that the mass of silver deposited was related to the pressure and also to the size of the silver anode ; the pressure effect was verified by Kahle, Richards, and Myers, and the latter observer found an increase when the liquid was saturated with nitrogen, but a decrease when the dissolved gas was carbon dioxide. Merrill repeated the pressure experiments and found no effect due to change of pressure alone. Lord Rayleigh and Mrs. Sidgwick observed an increase of deposit with increase of temperature; Richards, Collins, and Heimrod obtained greater deposits at 60°C. and at 0°C. than at 20°C. Leduc found a decrease with increasing temperature, and Merrill suggests that the mass is independent of the temperature. In more recent years Guthe and van Dijk have made a special study of various forms of voltameters. The form suggested by Richards was found by him to give a smaller deposit of silver than the form originally devised by Lord Rayleigh. The difference between the two forms found by Richards in 1899 was 80 parts in 100,000; in 1902 he found 44 parts in 100,000; Watson, in 1901, obtained a difference of 26; Guthe, in 1904, found 48, and in the same year van Dijk observed a difference of 23. van Dijk also compared the syphon and Rayleigh types and found a mean difference of 8 parts in 100,000, the latter form giving the heavier deposit; if a very doubtful observation is excluded, the mean difference is 18 parts in 100,000. In addition, van Dijk observed a difference due to the size of the platinum bowls, the smaller one invariably containing the lighter deposit for the same form of voltameter. There are many other interesting differences which need not now be enumerated; sufficient has been written to show that the silver voltameter could not be regarded as an instrument of high precision. The international ampere is, however, defined in terms of the deposit of silver, and the Conference on Electric Units at Charlottenburg in October, 1905, reaffirmed this definition, but expressed the opinion that the information before it was not sufficient to enable it to lay down exact directions in respect to the silver voltameter to be employed. Hence the necessity for an enquiry to ascertain the possibility of specifying a voltameter which is easily reproducable and in which an ampere-second always deposits the same mass of silver.