Page:Popular Science Monthly Volume 91.djvu/969

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Popular Science Monthly

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��the core, to get a low voltage from the magneto for the dash or tail lights. Use No. 20-gage wire for this also, and the num- ber of turns should be equal to the voltage

���Auto-transformer placed in the circuit to keep one light burning after the other burns out

required by the lights, multiplied by 17. The wiring for this is also shown in the diagram. — Claude Schudder.

��Constructing a Thermostat to Regulate Furnace Heat

IT is not always necessary, but is a con- venience to have a thermostat for regu- lating furnace heat. These instruments can be purchased at a nominal price; but one simple in construction can be made that will serve the purpose. Its working is based on the expansion of metals. The difference in the expansion of steel and brass is considerable. If thin strips of these two metals be firmly riveted together the rising temper- ature will cause them to bend, with the steel on the inside of the curve thus formed. This is due to the fact that brass ex- pands twice as fast as steel.

The accom- panying illustra- tion is almost self-explanatory. The two metals riveted together are shown at A, fastened to a block B. The short end of the lever C is connected at the other end of the metals by

���The unequal rate of ex- pansion of the metals causes the hand to move

��means of a silk thread, the long end being connected with the balance wheel D in a similar manner, the thread being given a few turns around the axle before it is fastened to a spoke. The pointer P is attached to the balance wheel and moves over a scale. The spring F is to return the pointer to its lowest reading.

Under varying temperature the pointer will assume various positions on the scale, which must be calibrated from a standard thermometer. It is only necessary to mark the scale between the temperatures re- quired for its action — from 60 to 80 deg. for a furnace.

To calibrate accurately it will be neces- sary to place a thermometer close to the instrument and adjust the pointer to the center of the scale when the temperature is at 70 deg. A mark is made on the scale. As the temperature changes, make the markings read in degrees on the scale.

By placing electric contacts on the pointer and also at knobs on the scale the thermostat may be used to operate electric control devices.

The steel and brass strips measure 3^8 in. long by ^2 in. wide. Each piece should be approximately .024 in. thick. Use small copper rivets for fastening them together. The wheels shown were taken from an old clock, which with their shafts and bearings made a fine working instrument. The old case was used also and cover plates put on to make a neat finish. — O. B. Hanson.

��An Inexpensive Multi-Point Loading Coil

NEARLY every amateur wireless en- thusiast has in his possession, or may readily obtain, the necessary materials for the construction of this loading coil. It is mounted in a box, which has a base 12 in. square and is 4 in. deep. The base of the box (the face of the mounted coil) is made of a single board. The knob K is the end of a ribbon spool about 2 in. in diameter and Yi in. thick. It is to be sand- papered and painted black, in imitation of hard rubber. For contacts, paper fasteners are used.

For coils, there is required about 1300 ft. of magnet wire. The number of coils is 50, each being 3 in. in diameter. For the rotary switch blade a piece of thin sheet .brass is used, and for binding posts two carbon terminals of worn-out dry cells were taken.

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