rts 



312 Professors Ayrton and Perry on 



find most convenient for measuring the resistance of any parts 

 of an electric-light circuit ; such as the field-magnet of a 

 dynamo, or the resistance of an arc-lamp while the strong cur- 

 rent is flowing. The thick coil is always kept, like an ammeter, 

 in the main circuit, and the terminals tt of the fine wire coil 

 are attached to any two points, the resistance between which, at 

 any particular moment, it is desired to know. The use of this 

 instrument also permits the employment of an iron wire, or 

 even of a bit of wet rope, as a resistance-coil for experimental 

 purposes, the resistance of the iron wire or of the wet rope 

 being determined with the ohmmeter at the moment the expe- 

 riment is being made. 



Sir William Thomson has recently proposed a new unit, a 

 " Mho," the reciprocal of an ohm, on account of the difficulty 

 which is found in making people realize that one thousandth 

 of an ohm may be of great importance when you are dealing 

 with a large current. But we do not propose graduating our 

 ohmmeters as mhometers, an operation of course quite simple 

 to effect ; because, first, if it be found that practical men can- 

 not be got to pay attention to a thousandth of an ohm, we 

 venture to think that they can probably be got to appreciate 

 its value if it be called a thousand microhms ; just as practical 

 men do not disregard the capacity of a mile of submarine 

 cable when called, as it always is, a third of a microfarad, 

 although, possibly, they might do so if it were spoken of as 

 a three millionth of a farad ; and, secondly, a still greater 

 objection to the adoption of the word "mho," ingenious as 

 is this suggestion of Sir William Thomson's (like all his 

 other suggestions), is that the constants in connexion with 

 conductivity, the reciprocal of resistance, are quite different 

 from those connected with resistance. For example, take the 

 case of increase of resistance with temperature. Let R be 

 the resistance of a wire at a temperature 2° C, and R that at 

 0°C; then R=R (l+O> 



where a is the increase of resistance per ohm per degree. Now 

 let C and C be the respective conductivities ; then 



C = Co(l-a'0, 

 where a! is the decrease per unit of conductivity per degree 

 of temperature. 1 



But since **'— q> 



and R = tt"? 

 ^o 



1 



\—at 



