THE REGULATION OF TEMPERATURE. 61 



3. The Master Relay. 



The master relay (R, Figure 29), so called because it controls all 

 the relays connected directly with the stoves, has a resistance of 250 

 ohms, and is of the type commonly used in telegraph lines. High 

 resistance in this relay is desirable in order to reduce to a minimum 

 the current which must pass through the thermostat. The course of 

 the only circuit which passes through the thermostat and the magnets 

 of the master relay is indicated in Figure 29 by the letter a. The 

 current in this circuit amounts to 8 milamperes plus, of course, what 

 may pass through the high-resistance lamp (h). 



4. The Minor Relay. 



The "local " of the master relay is made the " line " circuit of the minor 

 relay (Ri). The course of this circuit is indicated by the letter b. The 

 spark-gap of the master relay, like that of the thermostat, is spanned by 

 a lamp (k) and a condenser (c 2 ), although the latter is often dispensed 

 with. The minor relay has a resistance of only 20 ohms. Its spark-gap 

 is also spanned by a lamp (Z 3 ) and a condenser (c 3 ). The stove circuit, 

 as shown in the figure, passes through the " local" of the minor relay. 



In all of the relays — both master and minor — the usual arrangement 

 of the "stops" is reversed, so that the closing of the circuit through 

 the thermostat opens the circuit through the "locals" of the relays. 

 Obviously, what it really does is to cut down the current in these 

 circuits by throwing into them the resistance of the lamps (l 2 and k). 

 None of the three circuits employed in the system is ever fully broken. 

 But the currents which pass continuously through the lamps (h and l 2 ) 

 are insufficient to operate the relays, while that which passes continu- 

 ously through l 3 does not overheat the bath. 



By putting the minor relays in parallel, a single master relay is made 

 to operate any number of stoves. In some of our baths, the "master" 

 controls as many as eight stoves. 



For convenience in use, the system of electrical control is divided 

 into two units, and the apparatus belonging to each is permanently 

 installed on a portable board which may be fixed in any suitable 

 position with reference to a bath. All connections, except the perma- 

 nent ones on the boards, are made by means of flexible leads, to the 

 ends of which are attached insertion plugs. On the "master board" 

 are placed and wired together the master relay, the lamp which spans 

 the spark-gap of its "local," and plug attachments for the battery, 

 the condenser, and for several stove boards. On each of the minor 

 or stove boards are placed and wired together two minor relays in 

 parallel for as many stoves, the lamps which span their spark-gaps, 

 and plug attachments for the master board, the condensers, and the 

 stoves. The "unit" boards of each kind are uniform in arrangement 

 and size and are therefore interchangeable. 



