440 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
liable to grow up in an irregular manner, so as to meet the anode. In a 3-inch 
platinum bowl such a solution will allow of a current of about 1 ampere for a period of 
an hour. The strongest current which we have been able to use with a single volta¬ 
meter is about 2 amperes, and for this purpose we employed a solution containing 
one part of salt to two parts of water. It is probable that the deposit would have 
deteriorated if the current had been allowed to flow for much longer than a quarter of 
an hour, but in that time an ample amount (about 2 grms.) is obtained. The practical 
conclusion is that currents not exceeding 1^- ampere may be conveniently measured in 
a 3-inch voltameter by using a strong solution, and by stopping the operation after 
about a quarter of an hour. A shorter time than this would hardly allow of 
sufficiently precise measurement when a high degree of accuracy is aimed at. For 
purposes where an error of ^ per cent, is admissible, a duration of five minutes (300 
seconds) would be sufficient, and under these circumstances a stronger current would 
be unobjectionable. 
It will be seen that the application of this method to the measurement of such 
currents as are usually passed through incandescent lamps presents no difficulty, and 
we hope that it may be generally adopted as a control upon the indications of 
instruments depending for their trustworthiness upon the constancy of springs or of 
steel magnets. The anodes should be composed of fine silver sheet (about -§• inch 
thick), such as is sold for five shillings per ounce, and should not approach the sides of 
the bowl too closely, As there need be no waste of metal, the expense of silver as 
compared with copper should not be allowed to stand in the way of its use. For 
practical purposes it will be unnecessary to take some of the precautions which we 
thought incumbent upon us. After rinsing a few times with distilled water the 
deposit may be left to soak for an hour or so, and then after another rinsing dried over 
a spirit lamp. After the lapse of another hour it may be weighed, with a risk of error 
not exceeding a few tenths of a milligram. 
When still stronger currents have to be dealt with, the silver voltameter is less 
convenient. Platinum bowls of large size are not usually met with, but two or three 
may be combined in parallel without much trouble. In one of our experiments the 
same current was passed successively through a single voltameter, and through two 
arranged in parallel. The deposit in the single bowl, thrown down in 13 minutes, 
was 2 - 2327 grms. Those in the other bowls were P0114 and 1'2215, altogether 
2‘2329, agreeing almost precisely. In this way with three bowls, such as we have 
used, in parallel, there would be no difficulty in measuring currents up to 5 amperes. 
§ 28. The second branch of our subject is the evaluation of the electromotive force 
of standard galvanic cells. Enough has been said as to the means employed for 
measuring electric currents in absolute measure. If a current, after passing the 
current weighing apparatus, is made to traverse a known resistance, it will generate at 
the extremities of that resistance a known electromotive force. By suitably accom- 
