Thermostat for Controlling Temperature of a Room. 213 



and broken at these mercury surfaces, and the oil prevents 

 undue sparking and oxidation. 



M represents the electric heater, and L the two storage- 

 cells of the relay circuit. 



In the diagram the apparatus is represented at a moment 

 when the room has become slightly overheated, and the 

 heating circuit has been broken. As the air in the room 

 cools, the strip A deflects towards the left, until it makes 

 contact with the tip of the screw B x . Starting from A x we 

 may trace the circuit through the strip A, the screw B x , the 

 solenoids Ci and Hj, the mercury contacts K l5 and the battery 

 L, and then back to A x . The solenoid H x thus being 

 energized, causes the armature G x to be attracted, and so 

 pulls the beam F over to the left ; this causes the fork N to 

 bridge across the two mercury vessels P and P', thus com- 

 pleting the heating circuit. At the same time the relay 

 circuit is broken at K x by the withdrawal of the mercury 

 from the platinum wires K x , and a second relay circuit, 

 hitherto broken at K 2 , has that breach made good by the 

 mercury, and is ready to be completed so soon as a slight 

 incrense in the temperature of the room causes the strip to 

 move towards the right and make contact with B 2 . When 

 this takes place the solenoids C 2 and H 2 are energized, the 

 beam tips to the right thus breaking the heating circuit and 

 also the second relay circuit at K 2 . 



It will be realized that the two positions of the strip A, in 

 which it makes contact with B x and B 2 respectively, corre- 

 spond to two finitely different temperatures. The difference 

 between these temperatures, with the apparatus working at 

 about 15° C, is about o, l C, and corresponds to a linear 

 freedom of motion of about 0*25 mm. 



The reason for putting the solenoids C x and C 2 into the 

 circuits is to remove all possibility of an incomplete or 

 faltering contact between the strip and either of the contact 

 points B x and B 2 ; as soon as contact begins, a feeble current 

 is established in the relay circuit, and Oi or C 2 , as the case 

 may be, being slightly energized, attracts the invar of the 

 strip A (the invar we used has sufficient permeability) and 

 so increases the contact pressure, thus rapidly establishing 

 full contact. 



The mercury-tube K serves two purposes ; when the beam 

 tilts over, the mercury runs from one end of the glass tube to 

 the other, thus preventing the beam from rebounding ; at the 

 same time the fact that the relay circuit is broken at the 

 mercury contacts and not at the platinum contact points B x 

 or B 2 helps to prevent the latter from getting " oxidized." 



