450 Messrs. Scoresby and Joule on the Mechanical 



square feet, and the amalgamated zinc plate a surface of f rds 

 of a square foot. A pretty correct galvanometer, consisting 

 of a circle of thick copper wire and a magnetic needle 3 inches 

 long, was employed for measuring the currents of electricity 

 which were transmitted by the battery through the revolving 

 armatures. The tangents of the deflections of the magnetic 

 needle, corrected by a small equation, indicated the absolute 

 quantities of transmitted electricity. The quantity of zinc 

 consumed in the battery was deduced from the deflections of 

 the needle; the data of the calculation being derived from 

 previous experiments on the quantity of mixed gases evolved 

 from acidulated water by a current capable of producing a 

 given deflection of the needle. 



Our first experiments were made with the flat semicircular 

 revolving armature, its four coils being arranged for quantity. 

 The deflection of the needle before the engine was allowed to 

 start amounted to 64°, which indicated a current of 2232, 

 calling the current corresponding to 45°, 1000. The engine, 

 being then allowed to start, presently attained a velocity of 

 140 revolutions per minute. The needle was then observed 

 to stand steadily at 43°, indicating a current of 920. The 

 consumption of zinc in the battery was estimated to be at the 

 rate of 205 grs. per hour. 



Although we were not able to apply as exact a dynamo- 

 meter as we could have wished, we were nevertheless enabled 

 to arrive at a pretty correct estimation of the power developed, 

 by ascertaining the weight which, when thrown over a wheel 

 connected with the engine, was sufficient to keep it in uniform 

 motion. In this way we found that the force developed in 

 the above experiment was equal to raise 21,100 lbs. to the 

 height of a foot per hour. 



On making a second experiment with the same revolving 

 armature and battery, we obtained the following results: — 

 Current before the engine was allowed to start, 2232 ; current 

 when the armature was rotating at the rate of 180 revolutions 

 per minute, 850; consumption of zinc per hour, 190 grains; 

 force given out per hour, 17,820 lbs. raised a foot. 



Mr. J. P. Joule has already proved that the heat evolved 

 by voltaic and magneto-electrical currents is, cceteris paribus, 

 proportional to the square of their intensity*; and that the 

 power of the electro-magnetic engine is obtained at the ex- 

 pense of the heat due to the chemical reactions of the voltaic 

 battery by which it is worked. He has also shown, that if 

 the whole of the heat developed by the consumption of a grain 

 of zinc in a Daniell's battery could be converted into useful 

 * Phil. Mag., vol. xviii. p. 308, and vol. xix. p. 260. 



