current is stopped from flowing through the primary coil, that is on opening the key, the current induced in the secondary coil is in the same direction as that of the primary, but when the current is allowed to flow through the primary, as happens when the key is closed, the induced current travels in the opposite direction. If, then, we open and close the primary with great rapidity, say open one hundred times and close one hundred times per second, we obtain a short secondary shock with each opening and with each closing, or two hundred shocks per second. This rapid opening and closing is accomplished by a vibrating spring, which works automatically at the end of the instrument; and we graduate the strength of the shocks by increasing or diminishing the distance of the secondary coil from the primary, the shocks becoming weaker as we draw the secondary away from the primary.
The shocks from this instrument are much more irritating than those obtained from an ordinary battery. Thus, you see, I can hold the wires coming direct from the battery without being conscious of any irritation, but
