INDUCTION ELECTROMOTORS. 697 



received during the charge. The Commission found that the ac- 

 cumulators investigated, having absorbed 694,500 coulombs, gave 

 out 619,600. The efficiency in electricity is then 



619,600 



- - = 0-89. 



694,500 



This ratio does not give the yield in energy, since during the 

 charge the electricity enters the battery at a higher mean potential 

 than that which corresponds to the discharge. Let E, I, R, 

 be the electromotive force of the battery, the current, and the 

 resistance during the charge ; E', I', and R' the same quantities 

 during the discharge ; the difference of potentials at the terminals 

 is E + IR in the first case, and E' - I'R' in the second. The 

 quantities of electricity set free being Q and Q', the efficiency 

 expressed is 



Q'(E'-I'R') 



" Q(E + IR) ' 



It is always useful in practice to use a current for the charge 

 which is weaker than the discharge that is to say, to make !<!'; 

 we thus obtain a better result. 



In the experiments cited, the difference of potential at the 

 terminals were in the ratio of 2 to 3 ; it follows that 



Q'2 2 



'' 



1253. INDUCTION ELECTROMOTORS. Shortly after Faraday's 

 discovery, which was communicated to the Academy of Sciences 

 on Dec. 26th, 1831, Pixii, under the direction of Ampere,* con- 

 structed a machine intended to obtain induced currents by the 

 rotation of a magnet in face of an electromagnet. By the effect 

 of rotation, the current produced in the wire of the electromagnet 

 is of course alternating ; but by a commutator fixed on the axis of 

 rotation the currents in the external circuit are rectified. 



This first machine of Pixii has been modified in many different 

 ways. Clarke,f keeping the magnet stationary, caused the electro- 

 magnet to rotate; Pagej surrounded the magnet by a conducting' 



* AMPERE. Ann. de Chim. et de Phys. [2], Vol. LI., p. 71. 1832. 



f CLARKE. Phil. Mag, [3], Vol. ix., p. 262. 1836. 



PAGE. Ann. of Electricity, Magn., and Chemistry, Vol. in., p. 489. 1838-39. 



