110 ANNUAL OF SCIENTIFIC DISCOVERY. 



APPLICATION OF ELECTRICITY AND HEAT AS MOTIVE POWERS. 



AT the meeting of the British Association, Mr. Petrie stated, as the 

 result of careful experiments, that a voltaic current of one unit in 

 quantity (or that from one grain of zinc electro-oxidized per minute), 

 and of 100 degrees intensity, represents a dynamic force of 302^ 

 pounds raised one foot high per minute. From this we can infer an 

 important fact, that one horse-power is the theoretic or absolute dy- 

 namic force possessed by a current of electricity derived from the con- 

 sumption of one and fifty-six hundredth pounds of zinc per hour in a 

 Daniell's battery. But the best electro-magnetic engine that we can 

 hope to see constructed cannot be expected to give more than half or 

 a fourth of this power ; in any case, we see here the limit of power, 

 which no perfection of apparatus can make it exceed. Some of the 

 best electro-magnetic engines .ever invented, which have been tested 

 by Mr. Petrie and others on a practically useful scale, have only given 

 a power at the rate of fifty to sixty pounds of zinc per horse-power 

 per hour. The sinallness of this power in comparison with the ab- 

 solute value of the current (1.56 pounds zinc per horse-power per 

 hour) should not occasion surprise, if we consider the present state of 

 steam after many years of improvement. According to the determi- 

 nations of Joule and Rankine on heat, one pound of water raised 1 

 of temperature is equivalent to 700 pounds weight raised one foot. 

 But the best Cornish engines only yield one fourteenth of the power 

 that the combustion of the carbon actually represents, and many loco- 

 motives only one hundredth part, showing what great rewards may 

 yet await the exercise of inventive genius in this department, and that 

 we need not wonder that we have as yet only obtained one thirty-sec- 

 ond part of the power possessed by electricity. There is, however, a 

 far greater likelihood of obtaining a larger proportion of the real 

 power from electricity than from heat, owing to the character of the 

 two agents. If carbon could be burnt or oxidized by the air, directly 

 or indirectly, so as to produce electricity instead of heat, one pound 

 of it would go as far as nine pounds and one third of zinc (in a Dan- 

 iell's battery), chiefly because there are as many atoms in one pound 

 of carbon as there arc in five pounds and a quarter of zinc, and partly 

 because the affinity (for oxygen) of each atom of (incandescent) car- 

 bon is greater than that of an atom of (cold) zinc, minus the affinity 

 of the hydrogen for the oxygen in the water of the battery. 



The peculiar mode in which the electric current produces dynamic 

 effects has led to much miscalculation respecting the power obtain- 

 able from it. In any sort of an electric engine, the material to which 

 the neighbouring current gives motion, whether it be another movable 

 current, or, what is more usual, a magnetic body, is impelled in one 

 direction with a constant force, and this force, whether it be attrac- 

 tion, repulsion, or deflection, is, like the power of gravity, sensibly 

 constant at all velocities, provided only the same quantity per minute 

 of electric current be maintained. This is quite different in steam- 

 power, in which the faster the piston moves, the greater is the volume 

 of steam per minute that must be supplied to move it, or else the less 



