HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 389

nearly constant current is maintained. But such high resistanceinvolves a great loss in power, hence it is not practicable. Notso self-induction. Self-induction does not necessarily mean lossof power. The moral is, use self-induction instead of resistance.There is, however, a circumstance which favors the adoption ofthis plan, and this is, that a very high self-induction may heobtained cheaply by surrounding a comparatively small lengthof wire more or less completely with iron, and, furthermore, theeffect may be exalted at will by causing a rapid undulation of thecurrent, To sum up, the requirements for constant currentare: Weak magnetic connection between the induced andinducing circuits, greatest possible self-induction with theleast resistance, greatest practicable rate of change of thecurrent. Constant potential, on the other hand, requires : Clos-est magnetic connection between the circuits, steady inducedcurrent, and, if possible, no reaction. If the latter conditionscould be fully satisfied in a constant potential machine, its outputwould surpass many times that of a machine primarily designedto give constant current. Unfortunately, the type of machinein which these conditions may be satisfied is of little practicalvalue, owing to the small electromotive force obtainable and thedifficulties in taking off the current.

With their keen inventor’s instinct, the now successful arc-light men have early recognized the desiderata of a constantcurrent machine. Their arc light machines have weak fields,large armatures, with a great length of copper wire and fewcommutator segments to produce great variations in the current’sstrength and to bring self-induction into play. Such machinesmay maintain within considerable limits of variation in the re-sistance of the circuit a practically constant current. Their out-put is of course correspondingly diminished, and, perhaps withthe object in view not to cut down the output too much, a sim-ple device compensating exceptional variations is employed.The undulation of the current is almost essential to the commer-cial success of an arc-light system. It introduces in the circuit asteadying element taking the place of a large ohmic resistance,without involving a great loss in power, and, what is more im-portant, it allows the use of simple clutch lamps, which with acurrent of a certain number of impulses per second, best suitablefor each particular lamp, will, if properly attended to, regulateeven better than the finest clock-work lamps. This discoveryhas been made by the writer—several years too late.