Dr. Martin Jansson's Siderophone

a telephone trumpet t. When the primary circuit is closed, the enclosed iron bar d becomes magnetised, and in its turn it magnetises the smaller bar b. Secondary currents are thus induced in the fine coil around b. It will be observed that the prolonged axis of d cuts b at its middle ; two secondary currents of equal strength and running in opposite directions are thus induced in the fine coil surrounding b; these meet in the telephone trumpet and so long as they are equal in strength, produce no action there. When, however, the point of the inductor is approached to any small mass of iron x an alteration in the magnetic force of that half of the bar occurs, leading to an intensification of the induced current on the same side ; the overbalance of current will at once manifest itself b}7 a noise heard in the ear-trumpet. The instrument can be most accurately adjusted before use by means of the small screw shown in the drawing, since tliis screw contains