58 



Prof. D. E. Hughes. 



[May 15, 



its relative degree of increase ; and this measure may be expressed 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 induction 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 tele- 

 phone 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 switch- 

 ing key the telephone is instantly transferred to the sonometer, and 

 if its coil be afc zero, we should hear sounds when key is up or in con- 

 nection with the induction balance, and no sounds or silence when 

 the key is down or in connexion with the sonometer. If the sono- 

 meter coil was 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. 



During the course of these experiments with this instrument I 

 noticed that my own hearing powers varied very much with state of 

 health, weather, &c, that different individuals had wide differences of 

 hearing, and that nearly in all cases one ear was more sensitive than 

 the other ; thus whilst my degree of hearing was 10°, another might 

 be 60 in one ear and 15 in the other.* 



Dr. Richardson, F.R.S., who upon my invitation investigated this 

 subject, became so impressed with the value of the instrument as an 

 absolute measure of our hearing powers, and its capabilities of throw- 

 ing much light upon its relation with health, that he has undertaken a 

 series of researches which will extend over some time, and which I 

 think from some facts already gained will be of great value to the 

 medical profession. These experiments are now in his very able 

 hands, and he will in due time announce the results to the Royal 

 Society. 



If an observer's hearing is limited to 10°, how can we hear results 

 below this line ? I should have stated that when used to measure the 



* To this portion of my instrument when .used as a measurer of our hearing 

 powers, we have given the name of audiometer. 



