404 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 17 



With the resistance R^ cut out, the temperature of the furnace 

 rises and the boom of the galvanometer swings toward the left. 

 ^Vhen it reaches the left-hand contact it completes a circuit through 

 the polarized relay in parallel with the coils of the main relay C, and the 

 armature of the polarized relay moves over to the left hand contact. 

 This short circuits the coils of the main relay C, whose armature 

 drops back and restores the resistance R3 to the furnace circuit. 

 The action of the polarized relay B in short-circuiting the main relay 

 C stops the flow of current through its own coils and through the 

 left-hand contact of the galvanometer 



The furnace is now cooling, the relays are in their original positions, 

 and the apparatus continues to pass through repetitions of the same 

 cycle which differ only in the time element. 



DESCRIPTION OF THE REI.AYS, ETC. 



The galvanometer A, in this particular apparatus, was obtained 

 from the Weston Electrical Instrument Company ; it has an internal 

 resistance of about 50 ohms and a period of about ^/s second, and is 

 provided with two fixed contacts and one movable contact, said to be 

 of "iridium alloy." I have separated these contacts to allow room for 

 the ballistic swing, caused by the bell transformer, so that they close 

 at about plus and minus five millivolts. Experience has indicated 

 that the instrument might better have a resistance as low as 10 ohms 

 and a somewhat shorter period even if the sensibility were halved 

 by the change. The contact buttons need to be cleaned about once a 

 day, and polished with fine emery cloth, or a carborundum stone, 

 about once a month. 



The polarized relay B of the ordinary telegraph or burglar alarm 

 type is also provided with two fixed contacts. The coils are wound 

 with Number 40 B. & S. gage (O.OS mm.) copper wire to a resistance 

 of 2000 ohms in order to reduce the current through the galvanom- 

 eter contacts. 



The main relay C is of the ordinary telegraph type, wound to a re- 

 sistance of 150 ohms. It was found well worth while to replace the 

 original silver contacts of this relay, as well as those of the polarized 

 relay, with molybdenum. 



The bell transformer is of the size designed to operate an elec- 

 tric bell from a 110-volt lighting circuit. 



The "thermal lamp flasher" is a device which depends for its oper- 

 ation on the thermal expansion and contraction of a strip of metal 



