HIGH FREQ UENCY AND HIGH POTENTIAL CURRENTS. 337

plate p 2 . On tlie primaries are wound secondaries s and s 1? ofcoarse wire, connected to the devices d and l respectively. Byvarying the distances of the condenser plates c and c 1} and c andCj the currents through the secondaries s and Sj are varied inintensity. The curious feature is the great sensitiveness, theslightest change in the distance of the plates producing consid-erable variations in the intensity or strength of the currents. Thesensitiveness may be rendered extreme by making the frequencysuch, that the primary itself, without any plate attached to itsfree end, satisfies, in conjunction with the closed secondary, thecondition of resonance. In such condition an extremely smallchange in the capacity of the free terminal produces great varia-tions. For instance, I have been able to adjust the conditions sothat the mere approach of a person to the coil produces a con-siderable change in the brightness of the lamps attached to thesecondary. Such observations and experiments possess, of course,at present, chiefly scientific interest, but they may soon becomeof practical importance.

Very high frequencies are of course not practicable withmotors on account of the necessity of employing iron cores. Butone may use sudden discharges of low frequency and thus obtaincertain advantages of high-frequency currents without renderingthe iron core entirely incapable of following the changes andwithout entailing a very great expenditure of energy in the core.I have found it quite practicable' to operate with such low fre-quency disruptive discharges of condensers, alternating-currentmotors. A certain class of such motors which I advanced a fewyears ago, which contain closed secondary circuits, will rotatequite vigorously when the discharges are directed through theexciting coils. One reason that such a motor operates so wellwith these discharges is that the difference of phase between theprimary and secondary currents is 90 degrees, which is generallynot the case with harmonically rising and falling currents of lowfrequency. It might not be without interest to show an experi-ment with a simple motor of this kind, inasmuch as it is com-monly thought that disruptive discharges are unsuitable for suchpurposes. The motor is illustrated in Fig. 182. It comprises arather large iron core i with slots on the top into which are em-bedded thick copper washers c c. In proximity to the core isa freely-movable metal disc d. The core is provided with a pri-mary exciting coil c t the ends a and b of which are connected to