Balance and Sonometer. 153 



well, I think, to describe the nature of the apparatus for the 

 benefit of those who may not have read the paper I before 

 referred to. 



The induction-balance consists of two small wooden cups, 

 having a couple of spools, on each of which is wound about 100 

 yards of 36 silk-covered wire ; the lower bobbins are fixed, 

 and form the primary circuit ; the upper ones are adjustable, 

 and form the secondary circuit. 



If we now join the primary coils together, placing in then- 

 circuit a battery and microphone on which is placed a small 

 clock, and join the secondary coils together with a telephone 

 in its circuit, we shall, on listening, hear the clock ticking 

 by the action of the current induced in the coils. If we 

 now arrange the upper coils in such a way that they oppose 

 each other, we can, by adjusting the distance between them, 

 arrive at a zero-point at which we shall fail to hear the 

 faintest sound of the microphone. In this state the balance 

 is fit for use, for, if we introduce any metallic substance in 

 either of the cups, we immediately disturb the balance and 

 hear the clock ticking with an intensity of sound that 

 depends not only on the nature of the substance introduced, 

 but also by its weight or bulk. 



Suppose we place a piece of silver in one cup, it will 

 give out an intense sound ; and we find that we must place 

 in the other cup a piece of silver exactly alike, as regards 

 size, weight, standard, &c, to balance the coils that are to 

 produce the silence point again ; and so sensitive is the 

 instrument, that, with 24 Leclancher's cells, I have not yet 

 succeeded in balancing exactly any two silver coins, 

 although I used perfectly new ones for the experiment. 

 Gold ones were much easier to balance, using the same 

 battery ; but the sound given out by a gold coin is not 

 nearly so intense as that of silver, and I am inclined to 

 think that it will be necessary to use a larger battery 

 with different metals ; because, it appears to me, that, in 

 balancing any metals by sound, we of course depend upon 

 the delicacy of our sense of hearing. When the balance 

 arrives at nearly its zero-point we may not be able to dis- 

 tinguish any sound with, say, a battery of 20 cells ; but by 

 doubling the battery-power we increase the sound in the 

 microphone, and thus render the instrument more sensitive. 

 I should like to try the effect of a large battery, say, of 40 

 or 50 "Groves," on a balance, the primary coils of which were 

 wound with coarse wire, as I imagine this would detect 



