152 Hughes' Induction Currents 



If r 1 be the rate per cent, per annum yielded by the rent 

 of a piece of land, and r be the rate per cent, per annum of 

 the increment in its value — 



If V 1 be the price originally paid for land, V2 the value 

 after n years, then the value of r is obtained from the 

 equation — 



log. Y 2 - log. V, 



log. (1 + r ) = 



n 



If R be value of r obtained from this equation, then 

 R + r is the annual rate per cent, received by the investor 

 on his money. 



Art. IX. — Hughes' Induction Currents Balance and 

 Sonometer. 



By Robert E. Joseph. 



[Read 14th August, 1879.] 



On the 15th of May last Professor Hughes read before the 

 Royal Society, in London, a paper on " An Induction Currents 

 Balance, and Experimental Researches made with it;" and 

 the subject appeared to me to be of so much interest, and 

 capable of being utilised to a very great extent in physical 

 research, that I considered a few notes on the matter would 

 prove acceptable to the members of our own Society. 



Attempts have frequently been made to construct induc- 

 tion-balances for many years past ; but they do not appear to 

 have given satisfactory results. The introduction of the tele- 

 phone, however, as an instrument that will detect currents 

 of too feeble a nature to be detected by any other instru- 

 ment, has caused what before was found to be a difficulty 

 to now become quite easy, besides being simple and reliable. 



The instruments I introduce to your notice this evening 

 are the induction-balance, sonometer, clock-microphone, and 

 telephone, which, with a battery and reversing key, will 

 enable us to make any experiments we may require. Before, 

 however, proceeding with these experiments, it will be as 



