186 Mr. 0. Heaviside on the 



sands, millions, &c. of centimetres, according to the quite 

 arbitrary size of the instrument). It will then be sufficient 

 to find the places on the scale corresponding to 20, 21, 22, &c, 

 49, 50. Starting at 21, set the resistance-balance so that L 4 

 should be 21 units ; turn the movable coil till silence is 

 reached, and mark the place 21. Then set the balance to suit 

 22, turn again till silence comes, and mark again ; repeat 

 throughout the whole range. Why this can be done rapidly 

 is because the resistance-balance is at every step altered in the 

 same manner. We have thus an instrument of constant 

 resistance and variable known inductance, ranging from 



h + h — % m o to l x + l 2 + 2m 0) 



if l x and l 2 are the separate inductances and m the maximum 

 mutual inductance. The calibration is thoroughly practical, 

 as no table has to be referred to to find the value of a certain 

 deflection. 



I formerly chose 10 9 centim. as a practical unit of in- 

 ductance, and called it a torn ; the attraction this had for me 

 arose from L toms-r-B ohms equalling L/R seconds of time. 

 But it was too big a unit, and millitoms and microtoms were 

 wanted. Another good name is mac. 10 6 centim. might be 

 called a mac. Since Maxwell made the subject of self- 

 induction his own, and described methods of correctly mea- 

 suring it, there is some appropriateness in the name, which, 

 as a mere name, is short and distinctive. 



The two coils of the inductometer need not be equal ; but 

 it is very convenient to make them so, before calibration, by 

 the equal-ratio method, which, of course, merely requires us 

 to get a balance, not to measure the values. Let 1 and 2 be 

 any equal coils ; put one coil of the inductometer in 3, the 

 other in 4, and balance. It happened by mere accident that 

 my inductometer had nearly equal coils ; so I made them 

 quite equal, to secure two advantages. First, there is facility 

 in calculations ; next, the inductometer may be used with its 

 coils in parallel or in sequence, as desired. When in parallel, 

 the effective resistance and inductance are each one fourth of 

 the sequence values. Thus, let Y = ZC be the differential 

 equation of the coils in parallel, C being the total current, 

 and V the common potential fall ; it is easily shown that 



( ri + l lP )(r 2 + l 2 p 2 )-mY . Qn v 



r 1 + r 2 +(l ] + l 2 -2m)p > ' ' ' ( 60c > 



when the coils are unequal ; r x and r 2 being their resistances, 

 l x and l 2 their inductances, and m their mutual inductance in 



