A dynamo-electric machine (briefly termed a dynamo), suitably driven, forms a valuable source of supply where such must be instituted in the absence of, or independent of, a general supply. Thus, on board a ship in which electric light is not installed, in hospitals in isolated or country dis- tricts, or specially adapted for field service, this type of supply has much in its favour.

The dynamo may be constructed to supply current suitable for the special purpose in view, and some amount of regula- tion will be possible for variation of speed, though for each machine there is a certain rate of speed at which the greatest efficiency of action is obtained.

The choice of a special form of driving-power will depend mainly on the circumstances of the installation.

Where there is a pre-existent supply of power, with suffi- cient margin, the dynamo should, if possible, be driven from that, either by a direct chain or belt drive, or by way of an intervening countershaft, whereby variation of speed may be obtained, if that be desired.

Where no power-supply exists, and the installation is to be stationary, a small gas or oil engine will usually be the preferable power for driving. Special circumstances may make a steam-engine preferable. Where a sufficient water- power is available and convenient, an economical drive may be obtained from a water-turbine. This plan is highly com- mendable, wherever possible.

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