52 Prof. D. E. ITnfflios on an Induction-balance 



o 



tion-balances. If this secondary coil is near either primary 

 coil, we hear loud tones, due to its proximity. The same effect 

 takes place if the secondary coil is near the opposing coil, 

 except that the induced current is now in a contrary direction, 

 as a similar pole of the primary acts now on the opposite side 

 of the induction-coil. The consequence is, that as we withdraw 

 it from one coil and approach the other, we must pass a line 

 of absolute zero, where no current whatever can be induced, 

 owing to the absolute equal forces acting equally on both sides 

 of the induction-coil. This point is in the exact centre be- 

 tween the two coils. 



We thus possess a sonometer having an absolute zero of 

 sound : each degree that it is moved is accompanied by its 

 relative degrees of increase ; and this measure may be ex- 

 pressed in the degrees of the millimetres passed through, or 

 by the square of the distances in accordance with the curve of 

 electro-magnetic action. If we place in the coils of the in- 

 duction-balance a piece of metal (say copper, bismuth, or iron), 

 we at once produce a disturbance of the balance, and it will 

 give out sounds more or less intense on the telephone according 

 to the mass, or if of similar sizes, according to the molecular 

 structure of the metal. The volume and intensity of sound is 

 invariably the same for a similar metal. If by means of the 

 switching key the telephone is instantly transferred to the 

 sonometer, and if its coil be at zero, we hear sounds when 

 the key is up, or in connexion with the wire g, which leads 

 to the induction-balance, and no sounds, or silence, when the 

 key is down or in contact with the wire h, in connexion with 

 the sonometer. If the sonometer-coil were moved through 

 several degrees, or through more than the required amount, 

 we should find that the sounds increase when the key is 

 depressed ; but when the coil is moved to a degree where 

 there is absolute equality, if key is up or down, then the 

 degree on scale should give the true value of the disturbance 

 produced in the induction-balance ; and this is so exact that 

 if we put, say, a silver coin whose value is 115°, no other 

 degree will produce equality. Once knowing, therefore, the 

 value of any metal or alloy, it is not necessary to know in 

 advance what the metal is ; for if its equality is 115°, it is 

 silver coin ; if 52, iron ; if 40, lead ; if 10, bismuth ; and as 

 there is a very wide limit between each metal, the reading of 

 the value of each is very rapid, a few seconds sufficing to 

 give the exact sound-value of any metal or alloy. 



The respective values of the different metals may, as I have 

 already pointed out, be indicated by introducing the sono- 

 meter into circuit. I find, however, that it is difficult to 

 estimate fractional differences when the sounds to be com- 



