Popular Science Monthly 



953 



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. 



