HIGH FREQUENCY AND UIGII POTENTIAL CURRENTS. 377

ring of wrought iron 32 inches outside diameter, and about 1inch think. The inside diameter was 30 inches. There were 384polar projections. The wire was wound in zigzag form, but twowires were wound so as to completely envelop the projections.The distance between the projections is about T \ inch, and theyare a little over ^ inch thick. The field magnet was made rela-tively small so as to adapt the machine for a constant current.There are 384 coils connected in two series. It was found im-practicable to use any wire much thicker than Ho. 26 B. and S.gauge on account of the local effects. In such a machine theclearance should be as small as possible; for this reason themachine was made only 1J inch wide, so that the binding wiresmight be obviated. The armature wires must be wound with

great care, as they are apt to fly off in consequence of the greatperipheral speed. In various experiments this machine has beenrun as high as 3,000 revolutions per minute. Owing to the greatspeed it was possible to obtain as high as 10 amperes out of themachine. The electromotive force was regulated by means ofan adjustable condenser within very wide limits, the limitsbeing the greater, the greater the speed. This machine wasfrequently used to run Mr. Tesla’s laboratory lights.

The machine above described was only one of many suchtypes constructed. It serves well for an experimental machine,but if still higher alternations are required and higher efficiencyis necessary, then a machine on a plan shown in Figs. 204 to