188 Mr. 0. Heaviside on the 



inductometer in 6 and the other in 4, the coil to be measured 

 being in 3. Then 



L 3 =L 4 -2M 46 (33c) 



gives the induction-balance, L 4 being here the inductance of 

 the coil of the inductometer in 4, and M 46 the mutual induc- 

 tance of the two coils, in the position giving silence. This is 

 known in all positions, because the scale-reading gives the 

 value of l x + 1 2 + 2m (or else 2(l + m) if the coils are equal), 

 and l x + Z 2 is known. If the range is not suitable, we may, 

 as before, insert other coils of known inductance. 



There are other ways ; but these are the simplest, and the 

 equal-ratio method is preferable for general purposes. I 

 have spoken of coils always, where inductances are large and 

 small errors unimportant. When, however, it is a question of 

 small inductances, or of experiments of a philosophical nature, 

 needing very careful balancing, then the equal-ratio method 

 acquires so many advantages as to become the method. 



u So long as we keep to coils we can swamp all the irregu- 

 larities due to leading wires &c, or easily neutralize them, and 

 can therefore easily obtain considerable accuracy. With short 

 wires, however, it is a different matter. The inductance of a 

 circuit is a definite quantity : so is the mutual inductance of 

 two circuits. Also, when coils are connected together, each 

 forms so nearly a closed circuit that it can be taken as such ; 

 so that we can add and subtract inductances, and localize 

 them definitely as belonging to this or that part of a circuit. 

 But this simplicity is, to a great extent, lost when we deal 

 with short wires, unless they are bent round so as to make 

 nearly closed circuits, We cannot fix the inductance of a 

 straight wire, taken by itself. It has no meaning, strictly 

 speaking. The return current has to be considered. Balances 

 can always be got, but as regards the interpretation, that 

 will depend upon the configuration of the apparatus. 



" Speaking with diffidence, having little experience with 

 short wires, I should recommend 1 and 2 to be two equal 

 wires, of any convenient length, twisted together, joined at 

 one end, of course slightly separated at the other, where they 

 join the telephone wires, also twisted. The exact arrangement 

 of 3 and 4 will depend on circumstances. But always use a 

 long wire rather than a short one (experimental wire). If 

 this is in branch 4_, let branch 3 consist of the standard coils 

 (of appropriate size), and adjust them, inserting, if necessary, 

 coils in series with 4 also. Of course I regard the matter 

 from the point of view of getting easily interpretable 

 results." * 



* 'Electrician,' April 6, 1886, p. 490. 



