157 
reduces the current to one half of its amount when the 
engine is held stationary; and in this case the duty per grain 
of zinc is one half of the theoretical maximum. 
The same principles apply equally well when, instead of 
employing the machine as an engine evolving work, we do 
work on it by forcibly reversing the direction of its motion. 
Suppose for instance we urge it with this reverse velocity 
until the quantity of current is quadrupled or becomes 
22 - 336 indicated by a deflection of 87° 26'. Then we shall 
have 
1. Heat evolved per hour by the wire 442-64 x 4 2 = 7082-2 
units. 
2. Consumption of zinc per hour 2141 x 4 = 8564 grains. 
3. Heat due to 8564 grains of zinc 442-64 x 4 = 1770-56 units. 
4. Therefore the work per hour will be (1770-56 - 7082 - 2)772 = 
-4100432 ft. lbs. 
- . /*. ... . 4100432 i n, r> 
5. And the work per gram of zinc will be — e564~~ = ~ 478 -8 or 
- 3 times the maximum working duty. 
The principal reason why there has been greater scope 
for the improvement of the steam engine than for the 
electro-magnetic engine arises from the circumstance that 
in the formula - — -> applied to the steam engine by Thom- 
son, in which a and b are the highest and lowest tempera- 
tures, these values are limited by practical difficulties. For 
a cannot easily be taken above 459° + 374° =833° from 
absolute zero, since that temperature gives 12-425 atmos- 
pheres of pressure, nor can b be readily taken at less than 
the atmospheric temperature or 459° + 60°=519°. Also 
there is much difficulty in preventing the escape of heat ; 
whereas the insulation of electricity presents no difficulty. 
I had arrived at the theory of the electro-magnetic engine 
in 1840, in which year I published a paper in the 4th Vol. 
of Sturgeon’s Annals, demonstrating that there is “ no varia- 
