If the residual compound of zinc can be
utilized, and sold at a good price, then the cost of such motive power
may be reduced in proportion to the value of those by-products.
For the purpose of comparison, let us now employ the man who would
otherwise clean and prepare the primary cells, at engine driving. We let
him attend to a six horse power steam engine, boiler, and dynamo machine
for charging 50 accumulators, each of a capacity of 370 ampere hours, or
one horse power hour. The consumption of fuel will probably amount to 40
lb. per hour, which, at the rate of 18s. a ton, will give an expenditure
of nearly 4d. per hour. The energy derived from coal in the accumulator
costs, in the case of a supply of five electrical horse power for seven
hours, 2s. 9d.; the energy derived from the zinc in a primary battery,
supplying five electrical horse power for seven hours, would cost 17s.
3d.
It is hardly probable that any one would lay down a complete plant,
consisting of a steam or gas engine and dynamo, for the sole purpose of
charging the boat cells, unless such a boat were in almost daily use, or
unless several boats were to be supplied with electrical power from one
station.
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