Page:Life in Motion.djvu/62

scale or screen a beam of light from a lamp placed in front of the galvanometer. A very feeble current passing round this coil deflects the needle, and the deflection is seen by the movement of the spot of light, either to one side or to the other, according to the direction of the movement of the needle. These two large bobbins of fine wire form our induction coil. You observe they are not connected. Fig. 18.—Arrangement of apparatus for demonstrating Faradic currents, b, galvanic element; p primary and s secondary coil; g, galvanometer. In the experiment a reflecting Thomson galvanometer was used. In the circuit of the one we place a small battery and a key. In the circuit of the other we introduce the galvanometer. Watch the spot of light. When I close the key you observe an instantaneous movement of the spot of light. It swings to one side and then comes back, showing that the current passing through the galvanometer circuit is momentary. I now open the key, and you see again a momentary swing of the needle of the galvanometer, as