154 Hughes' Induction Currents 



even small differences in alloys which are not perceptible 

 in using a weak battery-power. 



For the purpose of making a perfect balance it is neces- 

 sary that both coils should be exactly alike as regards size 

 and length of wire ; it also requires that the substance to be 

 measured must be placed exactly in the centre between the 

 coils. I find you can produce a difference in the intensity of 

 the sound by varying the position of the substance, the 

 loudest being, as I before stated, when the article is in 

 between the centre of the coils. 



The sonometer, which is used in conjunction with the 

 balance, is similar in construction and action. 



It consists of two primary coils, sliding between which is 

 the secondary coil, the primary coils being wound so as to 

 oppose each other. It follows that if both coils were the 

 same as regards length and size of wire, the centre would be 

 the zero or silence point ; but in the one under your notice I 

 have wound them of different lengths, so as to obtain a wider 

 range of measuring. 



By means of this instrument we can obtain the exact 

 value of any person's hearing powers, which, as you will 

 presently see, varies very considerably. 



The use of this instrument alone appears to me to open 

 up a very interesting subject of research. I am unaware 

 of any other instrument that will enable us to ascertain and. 

 tabulate the strength of our hearing powers, from day to 

 day, or to compare the sensitiveness of our sense of hearing. 

 Suppose, now, we join the sonometer in circuit with the 

 balance, placing in its circuit a reversing key, so arranged 

 that we can switch-on either the balance or the sonometer. 

 If we now place any substance in the balance we get a 

 sound of a certain intensity; and by switching-in the 

 sonometer, and adjusting its secondary coil, until we 

 obtain a sound of similar intensity, we can read off 

 the value of that substance; and so sensitive and accurate 

 is its performance, that with a very little practice we are 

 enabled to detect the nature of the metal, although we need 

 not see it placed in the balance. Again, we may obtain the 

 sound value of a coin ; it will then be only necessary, sup- 

 posing any one were to place an unknown number of coins 

 in the balance, to obtain the sound value of the total 

 number, and we can easily calculate the nature and number 

 of coins placed in the balance. To ensure this expriment 

 being satisfactory, it is necessary to adjust both coils at a much 



