Instead of the magnet, a draught or blast of air may be employed with some advantage.  In this case the arc is preferably established between the knobs ab, in Fig. 2 (the knobs ab being generally joined, or entirely done away with), as in this disposition the arc is long and unsteady, and is easily affected by the draught.

When a magnet is employed to break the arc, it is better to choose the connection indicated diagrammatically in Fig. 5, as in this case the currents forming the arc are much more powerful, and the magnetic field exercises a greater influence.  The use of the magnet permits, however, of the arc being replaced by a vacuum tube, but I have encountered great difficulties in working with an exhausted tube.

[Illustration:  Fig. 5.—­Arrangement with low-frequency alternator and improved discharger.]

[Illustration:  Fig. 6.—­Discharger with multiple gaps.]

The other form of discharger used in these and similar experiments is indicated in Figs. 6 and 7.  It consists of a number of brass pieces cc (Fig. 6), each of which comprises a spherical middle portion m with an extension e below—­which is merely used to fasten the piece in a lathe when polishing up the discharging surface—­and a column above, which consists of a knurled flange f surmounted by a threaded stem l carrying a nut n, by means of which a wire is fastened to the column.  The flange f conveniently serves for holding the brass piece when fastening the wire, and also for turning it in any position when it becomes necessary to present a fresh discharging surface.  Two stout strips of hard rubber RR, with planed grooves gg (Fig. 7) to fit the middle portion of the pieces cc, serve to clamp the latter and hold them firmly in position by means of two bolts cc (of which only one is shown) passing through the ends of the strips.

[Illustration:  Fig. 7.—­Discharger with multiple gaps.]

In the use of this kind of discharger I have found three principal advantages over the ordinary form.  First, the dielectric strength of a given total width of air space is greater when a great many small air gaps are used instead of one, which permits of working with a smaller length of air gap, and that means smaller loss and less deterioration of the metal; secondly by reason of splitting the arc up into smaller arcs, the polished surfaces are made to last much longer; and, thirdly, the apparatus affords some gauge in the experiments.  I usually set the pieces by putting between them sheets of uniform thickness at a certain very small distance which is known from the experiments of Sir William Thomson to require a certain electromotive force to be bridged by the spark.