192 Mr. 0. Heaviside on the 



reason of the independence of the self-induction balance of 

 M 12 , we may, as before mentioned, wind them together, and 

 thus ensure their equality at every moment. There is then 

 only left the inequality between branches 3 and 4, which 

 must, of course, be separated for experimental purposes, and 

 that is very easily followed and set right. When a sound 

 comes on, holding a coin over the coil of lower resistance will 

 quench it, if it be slight and due to resistance inequality, and 

 tell us which way the inequality lies. If it be louder, the 

 cancelling will be still further assisted by an iron wire over or 

 in the same coil, or by a thicker iron wire alone, for reasons 

 to be presently mentioned. 



On the other hand, a small inequality in the inductance may 

 be at once detected by a fine iron wire, quenching the sound 

 w r hen over or in the coil of lower inductance : and when the 

 resistance- and induction-balances are both slightly wrong, a 

 combination of these two ways will show us the directions of 

 departure. These facts are usefully borne in mind and made 

 use of when adjusting a pair of coils to equality, during 

 which process it is also desirable to handle them as little as 

 possible, otherwise the heating will upset our conclusions and 

 cause waste of time. But a pair of coils once adjusted to 

 equality, and not distorted in shape afterwards, will practically 

 keep equal in inductance ; for the effect of temperature- 

 variation on the inductance is small, compared with the 

 resistance change. 



Regarding the intermitter, I find that it is extremely de- 

 sirable to have one that will give a pure tone, free from harsh 

 irregularities, for two reasons: first, it is extremely irritating to 

 the ear, especially when experiments are prolonged, to have 

 to listen to irregular noises or grating and fribbling sounds ; 

 next, there is a considerable gain in sensitiveness when the 

 tone is pure*. 



Coming now to the effects of metal in the magnetic field of 

 a coil, the matter is more easily understood from the theoretical 

 point of view in the first instance than by the more laborious 

 course of noting facts and evolving a theory out of them — a 

 quite unnecessary procedure, seeing that we have a good 

 theory already, and, guided by it, have merely to see whether 

 it is obeyed and what the departures are, if any, that may 

 require us to modify it. 



First, there is the effect of inductive magnetization in 

 increasing the inductance of a coil. Diamagnetic decrease is 



* I. e. pure in the common acceptation, not in the scientific sense of 

 having a definite single frequency, which is only needed in a special class 

 of cases, when no true balance could be got without it. 



