Page:Popular Science Monthly Volume 92.djvu/326

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��Popular Science Monthly

��fastened by its lower end in a vertical position to a wooden base 10 by 16 in. Near the upper end of the strip, and on either side, I mounted setscrews.

The distance between the upper end of the compound strip and the setscrews can thus be regulated as desired. The lower end of the strip, I wired to a battery and bell, then from both setscrews to the bell as shown. By this means, the bell is rung when the room becomes too warm or too cool. In either case, the circuit is made by the compound strip bending until it comes in contact with the set- screw. The dotted line shows the ther- mostat bent to the right because the heat in the room has expanded the brass faster than it did the iron. This rings the bell.

In the same way, when the brass con- tracts faster than iron, the strip is bent to the left.

The distance between the setscrew end and the strip can be regulated so that the bell rings at any desired temperature. A scale can be marked upon the wood back of the upper end of the strip, for convenience. This will vary for every thermostat made and should be deter- mined by using a thermometer. When the bell rings, I look at the thermostat to see whether fires need starting up or shutting off. The bell is easily silenced by slipping a piece of rubber between setscrew and strip. This breaks the circuit and the rubber falls as soon as temperature approaches normal and the strip comes back to its perpendicular position. When the iron strip is riveted to the brass, the two metals should be kept in the normal temperature, say at a temperature of 65 deg. for an hour before fastening them together. In this way the compound strip will always be perpen- dicular when the room is at about the right temperature. — F. E. Brimmer.

��Finding the Polarity of Electric Wires with a Potato

THE amateur electrical experimenter often finds it necessary to know which wire is the positive wire for making proper connections to his apparatus. The positive pole of a wire or of an electric battery can very easily be found by means of an ordinary potato. Cut the

��potato in half, lengthwise. For a low direct current voltage of 1^ volts to 10 volts, place the wires to be tested a short distance apart in the potato.

The positive pole part of the potato will turn green, while the negative one will remain colorless. This method can be used to find the polarity of one dry cell and upwards to 500 volts direct current; the only difference in using the higher voltage being that the wires require to be further apart. In using the 110 volts pressure, the time necessary to determine the polarity is about 10 seconds. With a current of say 2 volts and upward to 10 volts, it will require about one minute. If electrical workers will always carry a potato in their tool bag, they will have a reliable polarity indicator. The method cannot be used on alternating currents, as both ends of the wires would turn the potato green. — W. S. Standiford.

��A Spark Plug Tester with an Inclosed Spark Gap

AVERY simple and convenient tester for spark plugs on automobiles, motor boats, motor- cycles, etc., is shown in the accompanying illustration. The body of the tester is of hard rubber in which is imbedded the metal contacts, and the spark gap is protected by glass discs. The na- ture of engine trouble can be quickly diag- nosed by application of this tester. It shows at a glance, what otherwise might require an overhauling of the engine to dis- cover.

A regular spark shows a good plug. A non-spark shows a short circuit. An ir- regular spark shows defective porcelain. A clear spark shows that the trouble is in the mixture and indicates no power. A knock in the motor will be magnified, and the cylinder in which it occurs can be located. — Floyd L. Darrow.

���A spark gap is in- closed in the hard rubber tester

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