66 MR. J. P. JOULE ON THE LIFE AND WRITINGS 



Liebig, and also demonstrated by my own experiments, that 

 the production of force by the oxidation of zinc, is at least 

 one hundred times as expensive as that derived from the com- 

 bustion of coal in the steam engine. Therefore, admitting 

 that half the effect is wasted during the conversion of ordinary 

 force into that of current electricity, and also that half the 

 useful effect is wasted in the production of force by the voltaic 

 battery, we arrive at the conclusion that the zinc requisite to 

 produce a current of given intensity is at least twenty-five 

 times as expensive as the coal which, consumed in a steam 

 engine working a Sturgeon's magneto -electrical machine, 

 would be able to effect the same result. 



In an inquiry on the " Attributes of the Galvanometer," he 

 found that a voltaic pair, enfeebled by a long continued 

 action, is able to produce a powerful temporary current after 

 a few minutes rest. I have since been able to show that the 

 strong initial current which takes place on the first immersion 

 of a pair of zinc and copper plates into dilute acid is owing to 

 the presence of oxygen on the surface of the negative plate. 



In 1832 Mr. Sturgeon constructed an electro-magnetic 

 engine for turning machinery, which was the first contrivance 

 by means of which any considerable mechanical force was 

 developed by the voltaic current. Since that time, engines of 

 various forms have been constructed by Jacobi, Davidson, 

 and others ; but, as I have already observed, the hopes once 

 entertained of superseding steam as a motive force were found 

 to be fallacious. 



Professor Henry appears to have been the first who ob- 

 served a spark on the disruption of a voltaic circuit in which 

 a long wire or coil was included. This phenomenon he 

 justly attributed to the dynamical induction discovered by 

 Dr. Faraday. Mr. Jenkin observed that on placing a bar of 

 iron within the coil, the intensity of the electrical action on 

 the breaking of the circuit is sufficient to produce a shock. 

 Henry and Faraday extended the subject still further, by 



