Page:Popular Science Monthly Volume 89.djvu/968

954

HE device described comprises an actuating lever to operate the draft and check; a relay—a skeleton bell

or buzzer—to release the lever, and a clock to close the electrical circuit at the proper hour. A thermostat may also be used and connected in the circuit. In addition, there is a trigger intervening between the relay and the lever, and a circuit-breaker which effectively prevents loss of power by short-circuiting the battery after the mechanism has been tripped.

Reference to the sketch will show that the circuit-closer consists simply of an ordinary alarm clock, back of which extends upward a metal strip which makes contact with the winding key in revolution as the alarm goes off and forms a circuit through the clock and a turn or two of bare wire on which the clock rests and which leads to the other terminal. The relay is short-circuited so that the armature does not buzz but is simply drawn sharply up to the magnets, thus pulling the trigger lever forward by the wire connection and releasing the lever by raising the bent wire stop.

As the lever falls it not only opens the draft and closes the check, but pulls a third cord which opens a small battery-switch connected between relay and clock, thus preventing current waste should the winding key chance to remain in contact with the metal strip when the alarm has ceased ringing.

It was found that the check-valve on the furnace in question must be lowered, not raised, to make it draw. The detail sketch shows how this difficulty was overcome by providing a stout wire support for the check-valve which is jerked loose as the lever falls, thus allowing the check to drop to its closed position by its own weight. On a single cell of battery the device will be found to work perfectly.—

MONG the many sources of trouble by which a radio set's efficiency is reduced, a short-circuit of a few turns in the primary of the transformer is to an inexperienced man one of the most difficult to locate, as well as to find a remedy for. With the primary of the transformer short-circuited, two fundamental principles will explain the cause for the lack of efficiency:

1. In order for the greatest efficiency to be obtained, there must be resonance between the primary circuit and the secondary circuit. It is evident that, if some of the turns are cut out of the primary of the transformer, the two will be thrown out of resonance, since the electrical inductance of the primary will be reduced. This, of course, results in poor working.

2. The other fundamental principle is explained by the potential drop in a circuit. Suppose a circuit, as shown, to be partially short-circuited so as to cut out three turns of the nine turns

which constitute the primary of the transformer, as from A to B. The total electromotive force and the internal resistance will remain constant, but the external resistance and current are