"96 Rayleigh's Address to the British Association. 



should naturally have felt inclined to dilate upon it, but that I 

 feel it to be too abstruse and special to be dealt with in detail 

 upon an occasion like the present. As regards resistance, I will 

 merely remind you that the recent determinations have shown a 

 so greatly improved agreement, that the Conference of Electricians 

 assembled at Paris, in May, felt themselves justified in defining 

 the ohm for practical use as the resistance of column of mercury 

 of ° C.j one square millimetre in section, and 106 cent- 

 metres in length — a definition differiDg by a little more than one 

 per cent, from that arrived at twenty years ago by a committee of 

 this Association. 



A standard of resistance once determined upon can be embodied 

 in a "resistance coil," and copied without much trouble, and 

 with great accuracy. But in order to complete the electrical 

 system, a second standard of some kind is necessary, and this is 

 not so easily embodied in a permanent form. It might conve- 

 niently consist of a standard galvanic cell capable of being pre- 

 pared in a definite manner, whose electro-motive force is once for 

 all determined. Unfortunately, most of the batteries in ordinary 

 use are for one reason or another unsuitable for this purpose, but 

 the cell introduced by Mr. Latimer Clark, in which the metals 

 are zinc in contact with saturated zinc-sulphate and pure mercury 

 in contact with mercurous sulphate, appears to give satisfactory 

 results. According to my measurements, the electro-motive 

 force of this cell is 1,435 theoretical volts. 



We may also conveniently express the second absolute electrical 

 measurement necessary to the completion of the system by taking 

 advantage of Faraday's law that the quantity of metal decomposed 

 in an electrolytic cell is proportional to the whole quantity of 

 electricity that passes. The best metal for the purpose is silver 

 deposited from a solution of the nitrate or of the chlorate. The 

 results recently obtained by Professor Kohlrauseh and by myself 

 are in very good agreement, and the conclusion that one ampere 

 flowing for one hour decomposes 4.025 grains of silver, can 

 hardly be in error by more then a thousandth part. This number 

 being known, the silver-voltameter gives a ready and very accu- 

 rate method of measuring currents of intensity, varying from 1-10 

 ampere to four or five amperes. 



The beautiful and mysterious phenomena attending the 



