310 



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 



