ON STANDARDS OF ELECTRICAL RESISTANCE. 127 
371 feet, and the latter to 1217 feet, of pure copper wire ;/;th in, in diameter 
at 15°C. Both of these units have been adopted in scientific experiments 
and in practical tests; and it was thought that the absolute magnitude of 
the unit to be adopted should not differ widely from these resistances. 
The importance of the second quality required in the unit, that of forming 
part of a coherent system of electrical measurements, is felt not only by 
purely scientific investigators, but also by practical electricians, and was 
indeed ably pointed out in a paper read before this Association in Manchester 
by Sir Charles Bright and Mr. Latimer Clark, 
The Committee has thus found itself in the position of determining not 
only the unit of resistance, but also the units of current, quantity, and electro- 
motive force. The natural relations between these units are, clearly, that a 
unit electromotive force maintained between two points of a conductor 
separated by the unit of resistance shall produce the unit current, and that 
this current shall in the unit of time convey the unit quantity of electricity. 
The first relation is a direct consequence of Ohm’s law ; and the second was 
independently chosen by Weber and by the two electricians above named. 
Two only of the above units can be arbitrarily chosen; when these are 
fixed, the others follow from the relations just stated, 
Sir Charles Bright and Mr. Latimer Clark propose the electromotive force 
of a Daniell’s cell as one unit, and choose a unit of quantity depending on 
this electromotive force. Their resistance-unit, although possessing what we 
have called the second requisite quality, and superior consequently to many 
that have been proposed, does not in any way possess the third quality of 
bearing with its co-units a definite relation to the unit of work, and has 
therefore been considered inferior to the equally eoherent system proposed 
by Weber many years since, but until lately comparatively little known in 
this country, 
Professor Weber chose arbitrarily the unit of current and the unit of 
electromotive force, each depending solely on the units of mass, time, and 
length, and consequently independent of the physical properties of any arbi- 
trary material. 
Professor W. Thomson has subsequently pointed out that this system 
possesses what we have called the third necessary quality, since, when defined 
in this measure, the unit current of electricity, in passing through a conductor 
of unit resistance, does.a unit of work or its equivalent in a unit of time*, 
The entire connexion between the various units of measurement in this 
system may be summed up as follows, 
A battery or rheomotor of unit electromotive force will generate a current 
of unit strength in a cireuit of unit resistance, and in the unit of time will 
conyey a unit quantity of electricity through this circuit, and do a unit of 
work or its equivalent, 
An infinite number of systems might fulfil the above conditions, which 
leave the absolute magnitude of the units undetermined, 
Weber has proposed to fix the series in various ways, of which two only 
need be mentioned here—first by reference to the force exerted by the current 
on the pole of a magnet, and secondly by the attraction which equal quantities 
of electricities exert on one another when placed at the unit distance, 
In the first or electro-magnetic system, the unit current is that of which the 
unit length at a unit distance exerts a unit of force on the unit magnetic 
pole, the definition of which is dependent on the units of mass, time, and 
* Vide * Application of Electrical Effect to the Measurement of Electromotive Force,” 
Phil. Mag. 1851. 
