Page:Electrical Engineering Volume 1.djvu/356

1788 located under the seats or in lockers, and is usually of sufficient size to furnish current for running the boat at a speed of seven or eight miles an hour for about 40 miles; higher rates of speed can be obtained, but the total distance covered is then lessened.

2807. In general, accumulators have been more or less successfully applied (a) where it is desired to supply a variable demand for current and at the same time keep the output of the source of the power approximately constant; (b) where it is desired to utilize an electric current at a point where it is objectionable or impossible to obtain the current directly from dynamo machines; (c) where it is desired to obtain a comparatively small but continuous current from a source of a considerable current, which can be utilized only a short time and at infrequent intervals, and (d) where a perfectly steady current is required for certain applications, where the current from dynamo machines, which is always slightly irregular, would be unsuitable.

2808. Under (a) would be classed the various lighting and power station installations, the principles of which have been described (Art. 2792). An extension of the plan of such stations has been adopted abroad, in cases where a considerable demand for current from a (direct current) central station occurs in some particular district, at a distance from the station. It is evident that if this demand is met by sending the current directly from the station, the wires for carrying the current must be large enough to carry the maximum current required, although this maximum only continues for a few hours in each day. If an accumulator plant be installed in this district, the wires from the station need only be large enough to carry the average current required for that district, the battery furnishing the additional current during the period of heavy load and charging when the load is less than the average, just as in the first plan for installing accumulators in central stations. (See Arts. 2792 and 2793.) Aside from the saving in the wire,