Seeing an Alternating Current in a Mirror
It will almost appear impossible to those unfamiliar with laboratory methods that one may watch the vibrations—3,600 per minute—of an alternating current in a little pocket mirror without the use of any apparatus other than a telephone receiver. The experiment is very interesting and instructive, one that may be performed at practically no expense.
The Alternations of the Current may be Seen by Looking in the Mirror
Take an ordinary inexpensive watchcase receiver, drill a hole in the cover for a short piece of brass tubing, to make a gas connection, and then plug up the center opening with a cork, into which is tightly fitted a piece of ¹⁄₈-in. tubing. The upper end of this should be closed with a plug having a central opening about the size of a pin. Procure a small rectangular pocket mirror and remove the celluloid covering, and then, across the back, solder a piece of straight wire to form a vertical spindle, about which the mirror may be rotated. Connect any resistance, such as a magnet coil of 10 or 20 ohms, in series with an incandescent lamp, and then connect the receiver terminals to the ends of this resistance. In this manner an ideal alternating-current supply of a few volts to operate the receiver safely is secured. Turn on the gas only sufficient to produce a narrow pencil of flame, not over 1 in. long. Mount the mirror as shown, or hold the spindle between the thumb and forefinger of the left hand while rocking it back and forth with the right. Ordinarily only a streak of light will appear, but immediately upon turning on the current this streak will be broken up into a series of regular waves, flatter or sharper according to the speed with which the mirror is rocked. After carefully noting the wave form, connect the receiver with the primary of an ordinary medical coil, across the make-and-break, and note the marked difference in the waves.
By replacing the receiver with a block of wood having a circular depression, about 2 in. in diameter and ¹⁄₈ in deep, over which is pasted a disk of smooth paper, the waves set up by the human voice may be observed if the talking is done loudly and close to the disk. The gas connection in this case is made from the back of the block, as shown. As the several vowels are sounded, the characteristic wave from each will be seen in the mirror. It is also interesting to increase the pitch of the voice and note how much finer the waves become.