DRIVES AND CONTROLS 



235 



, Copper block 



li^gtegiJ 



m 



\smm^ 



I Bimei-allic slrip 



^,' Make and 

 break 

 coniach 



/ Copper block 

 ^^-/Heal barriers 



/ U"'; Bimelalllc 



^I^jf - ^^'''P^ 



Make and break conlacis 

 Fig. 480. — Thermal relays. Inexpensive time delay 

 utilizing the effect of a heating coil around a bimetallic 

 strip. Least accurate device. Has a slow make and break 

 action. For longer time intervals, a copper block may be 

 mounted to absorb some of the heat; the larger the block of 

 copper, the longer the time interval. Time intervals 

 ranging fromi J^ sec. to 5 to 10 min. are possible with this 

 de\ace. 



ff 



Copper Jackel or ring or ' 

 shorl circuiled furns 



t« 



m 



Moke and break 

 conlacis 



«iir^ 



Fig. 481. — Magnetic time delay, used on direct 

 current only. Relatively inexpensive, effects 

 time delays up to 10 sec. by means of residual 

 magnetism. Magnet may be copper jacketed, 

 may have copper rings, or may have short-cir- 

 cuited turns around the magnet. Variation in 

 the amount of copper or in the resistance of short- 

 circuited turns will affect the time delay. 



Moke and break 

 con tads 



\y Solenoid 



Eleclro magnel' 



Fig. 482. — Magnetic-drag time delay. 

 A small electromagnet is used, and the 

 motion of the relay plunger is made to 

 revolve a metal disk in the field of the mag- 

 net. The rotation of the disk is retarded by 

 magnetic induction. Reliable device, trou- 

 ble free, but relatively expensive. 



