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Page:Popular Science Monthly Volume 90.djvu/649

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Popular Science Monthly

��633

��altogether. With both switchblades at the left, in positions W and F, the condenser is in series, as shown in Fig. 5. When both switchblades are thrown to the right, in positions X and Z, the condenser is in shunt to the primary coil, as in Fig. 4. When the upper switchblade is at the right and the lower at the left, i.e., in positions X and Y, the variable condenser is cut out and the circuits are connected as shown in Fig. 2. This same switching arrangement may also be used with the autotransformer arrangement of Fig. i . Practically the same results will be secured, except that the pri- mary turns are changed whenever the switch C (Fig. i) is moved to vary the coupling.

Experienced radio operators, as well as beginners, will find it worth while to study the principles explained in this article. They are the basis of successful operation of the coupled receiving sets. {The end)

��Power Measurement Method Without a Watt-Meter

MANY amateur electricians would like to know the power consumption of devices they construct, but do not have access to a watt-meter. The watts con- sumed may be found as follows: Turn ofif all lights in the house except one of known power consumption, and count the number of seconds required for the watt-hour meter's disk to make one revolution. Then turn out all lights, connect into the circuit the device whose power consumption you desire to know, and again count the number of seconds for the meter disk to make one revolution.

To find the power consumed by the device, multiply the watts of the lamp by the number of seconds required for a revolu- tion of the meter disk when connected and divide by the number of seconds required for a disk revolution with the device itself connected. For example, one meter re- quired 30 seconds for one disk revolution with a 25-watt lamp connected, and 60 seconds with a small transformer furnishing current to a small motor connected with it. The power consumption of the transformer and motor was thus 25 X 30 4- 60 = 12.5 watts. If the disk runs so fast that it is diffi- cult to time it for one revolution, the time for a greater number of revolutions should be taken, the number being the same with the device tested as with the lamp.

��An Easily Constructed Detector-Stand for Wireless Instruments

A DETECTOR-STAND can be easily made from a few simple materials. Procure a piece of brass 1/16 in. thick, and cut it into the shape shown at A. The openings may be made by drilling holes and

���A detector-stand which can be easily made by the amateur from a few simple materials

filing the edges. Bend the brass on the dotted lines so that a U-shaped piece is formed with the slots parallel.

Obtain a brass or copper ball l^ in. in diameter, and drill a hole through it to admit a brass rod i^ in. long. Thread one end of the rod to fit the insulating knob. Solder the ball and a piece of fine brass wire on to the rod, as shown at C. The point of a gold scarf-pin may be soldered on to the end of the wire to serve as a "cat- whisker." It is easily adjusted.

The standard B is heavy brass, bent on the dotted lines. The cup, with the crystal, is fastened to it with a screw as shown. The inverted position of the crystal prevents dust from settling on it and maintains its sensitiveness. The base may be made of

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��Construction diagram of the home-made detector-stand showing dimensions

��hard rubber or any hard wood. Be sure that the uprights of the piece A are just tight enough to hold the ball in place. The knob can be moved backward or forward, or in any direction desired to secure the best crystal setting. — G. F. Exner.

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