X-Rays, Apparatus and Methods Page 6

changed. To remedy this, we have devised a form of switch shown in the diagram, Fig. 3, by means of which the condenser capacity may be shifted, at will and instantly, by simply turning the switch. The effect upon the secondary discharge is very marked, both volume and musical note of the spark changing with the position of the switch. We find this idea exceedingly convenient in general experi- mental work with alternating currents, it being possible with it to alter condenser capacity as quickly and as readily as we may self-induction or resistance. (We have made some experiments in the use of a condenser in parallel with the primary. The volume of the secondary discharge seems, in many cases, to be greatly increased.)

In arranging coils to operate from few or many cells of battery it is, of course, merely a matter of winding the primary with coarse or fine wire, the number of ampère turns being kept the same. It is interesting to note, as we have noted experi- mentally many times, that the larger the primary E.M.F. the smaller must be the condenser capacity employed, and vice versa. This is, of course, only to be expected, since capacity and self-induction are inverse functions of each other.

Primary Current.—This may be either from primary batteries, storage batteries, or from a commercial direct-current circuit. Primary batteries have too high resistance to give sufficient current for coils of much size—say 4 inches and over — unless joined in parallel, and then we need a number of such groups to secure the requisite E.M.F. They require constant attention, soon run down, and are expensive to operate. Storage batteries are exceedingly satisfactory, are comparatively inexpensive to operate, and require practically no attention. Charging them is more or less of an annoyance, however, especially where they must be sent