153 
copper wire 389 feet long, and Ar of an inch diameter, or 
offering a resistance equal to one BA unit. Then, on con- 
necting the terminals of this wire with the battery, and 
keeping the engine still, the current through the wire will 
be such as, with a horizontal force of earth’s magnetism 3 - 678, 
would be able to deflect the small needle of a galvanometer 
furnished with a single circle of one foot diameter, to the 
angle of 54° 23'. Also this current going through the above 
wire for one hour will evolve heat that could raise 110-66 lbs. 
of water 1°, a quantity equal to 85430 ft. lbs. of work. In 
the meantime the zinc consumed in the battery will be 
535 - 25 grains. Hence the work due to each grain of zinc 
is 159 - 6 ft. lbs., and heat -20674 of a unit. 
I. In the condition of the engine being kept still we have 
therefore, current being 1-396, as shown by a deflection of 
54° 23', 
1. Heat evolved per hour by the wire 110-66 units. 
2. Consumption of zinc per hour 535 - 25 grains. 
3. Heat due to 535-25 grains, 110-66 units. 
4. Therefore the work per hour will be(110 - 66- 1 10 ’66)7 7 2 = 0. 
5. And the work per grain of zinc will be ^§^25 ~ 
II. If the engine be now started and kept by a proper 
load to a velocity which reduces the current to §, or -9307, 
indicated by deflection 42° 57' we shall have 
1. Heat evolved per hour by the wire 1 1066 x 
2 ' 
2. Consumption of zinc per hour 535-25 x ~ = 356-83 grains. 
3. Heat due to 356-83 grains, 110-66 x g = 73-77 units. 
4. Therefore the work per hour will be (73-77-49-18)772 
= 18983 ft. lbs. 
18983 
5. And the work per grain of zinc will be -g3 - = 53 - 2 or } of 
the maximum. 
III. If the load be lessened until the current is reduced 
to \ of the original amount, or to -698, we shall have 
{§} 
= 49 - 18 units. 
