156 
IV. Let the load be further reduced until the velocity 
reduces the current to or to 1'8G13 shown by a deflection 
of 61° 45'. Then we shall have 
1. Heat evolved per hour by the wire 442'64 x - 49T82 units. 
2. Consumption of zinc per hour 2141 x - = 713'66 grains. 
3. Heat due to 7 13-66 grains of zinc 442 - 64 x i= 147-55 units. 
O 
4. Therefore the work per hour will be (147 '55 - 49-182)772 = 
75940 ft. lbs. 
75940 
5. And the work per grain of zinc will be — 106'4 or § of 
the maximum. 
V. Let the load be still further reduced until, with the 
increased velocity, the current becomes reduced to tott, or to 
•05584 showing a deflection of 3° 12'. Then w T e shall have 
1. Heat evolved per hour by the wire 442 - 64 x =-044264 
of a unit. 
2 . Consumption of zinc per hour 2141 x jqq = 21 - 41 grains. 
3. Heatdueto 21-41 grains of zinc 442-64 x = 4-4264 units. 
4. Therefore the work per hour will be (4‘4264 - -04426)772 = 
3383 ft. lbs. 
3383 
5. And the work per grain of zinc will be = 158 or of 
the maximum duty. 
Now suppose that we still further improve our engine by 
# 
making the stationary magnets twice as powerful. In this 
case all the figures will remain exactly the same as before, 
the only difference being that the engine will only require 
to go at half the velocity in order to reduce the current to 
the same fraction of its first quantity. The attraction will 
be doubled, but the velocity being halved no change will 
take place in the amount of work given out. 
In all cases the maximum amount of work per hour is 
obtained when the engine is going at such a velocity as 
