Self-induction of Wires, 211 



evident. But wires 1 and 2 must be thoroughly well twisted, 

 before being wound into a coil, if it is desired to get rid of 

 the influence of, say, M 61 and M 62 , when it is a coil that 

 operates in 6, and this coil is brought near to 1 and 2. [This 

 leads me to remark that a simple way of proving that the 

 mutual induction between iron and copper (fine wires) is the 

 same as between copper and copper, which is immensely more 

 sensitive than the comparison of separate measurements of the 

 induction in the two cases, is to take two fine wires of equal 

 length, one of iron, the other of copper, twist them together 

 carefully, wind into a coil, and connect up with a telephone 

 differentially. On exposure of the double coil to the action 

 of an external coil in which strong intermittent currents or 

 reversals are passing, there will be hardly the slightest sound 

 in the telephone, if the twisting be well done, with several 

 twists in every turn. But if it be not well done, there will 

 be a residual sound, which can be cancelled by allowing in- 

 duction between the external or primary coil and a turn of 

 wire in the telephone -circuit. A rather curious effect takes 

 place when we exaggerate the differential action by winding 

 the wires into a coil without twists, in a certain short part or 

 its length. The now comparatively loud sound in the telephone 

 may be cancelled by inserting a nonconducting iron core in 

 the secondary coil, provided it be not pushed in too far, or go 

 too near or into the primary coil. This paradoxical result 

 appears to arise from the secondary coil being equivalent to 

 two coils close together, so that insertion of the iron core does 

 not increase the mutual inductance of the primary and secon- 

 dary in the first place, but first decreases it to a minimum, 

 which may be zero, and later increases it, when the core is 

 further inserted. Reversing the secondary coil with respect 

 to the primary makes no difference. Of course insertion of 

 the core into the primary always increases the mutual induc- 

 tance and multiplies the sound. The fact that one of the 

 wires in the secondary happens to be iron has nothing to do 

 with the effect.] 



Another way of getting unions of the two conditions of the 

 induction-balance is by having branches 1 and 3 equal, instead 

 of 1 and 2. Thus, if we take R 1 = R 3 , L! = L 3 , R^R^ in A 2 

 and A 2 (73c) and (74c), we obtain fifteen sets of double con- 

 ditions similar to those already given, out of which just four 

 (as before) unite the two conditions. Thus, using M 13 only, 

 we have 



L 2 = L 4 , (101c) 



and the same if we use M 24 only, and the same when both M l3 



