OCT. 19, 1921 ROBERTS: FURNACE REGULATOR 407 



boom, while the strip between the parallel lines represents the zone 

 in which the boom may lie without causing a movement of the switch. 

 The black dots indicate the positions of the boom at the instants 

 when the mechanically operated contact mechanism is depressed. 

 The ratio of heating time to cooling time in the upper diagram is 13 :12 

 and there is a slow, cyclic variation in temperature extending over 

 25 of the equal time interv^als. In the lower diagram, where the ratio 

 is 19:31, this slow variation is much less serious. Decreasing the 

 time interval between the successive instants at which contact can 

 be made \vould undoubtedly improve matters greatly, even if the 

 period of the galvanometer could not be decreased."* 



The galvanometer with fixed contacts, as used in the present ap- 

 paratus, is free from this restriction and may make contact at any time. 

 Neither type of galvanometer follows the temperature of the heater 

 very closely; the inertia of the moving coil causes a considerable 

 lag which, however, seems to do no harm. 



The regulator has thus far been used only on direct current furnished 

 by a battery or generator. The pulsating direct current furnished 

 by a mercury arc rectifier causes the galvanometer to vibrate seri- 

 ously because of the alternating current induced in the bell trans- 

 former by the pulsations. Several plans have suggested themselves 

 to make it operate on alternating current: partially rectifying the 

 current with an aluminum cell; putting a 6-volt storage battery in 

 series with the source of suppl}^; or making an alternating-cinrent 

 galvanometer. In any case, it would be necessary to operate the bell 

 transformer from a separate source of direct current, or to adopt some 

 other scheme, such as rotating the galvanometer contacts, in order 

 to prevent their sticking. The alternating-current galvanometer 

 has seemed most hopeful and one is now under construction. 



Although the foregoing discussion has confined itself to discussing 

 the regulator in combination with a particular type of furnace, there 

 is nothing to prevent the same principles from being applied to other 

 purposes, such as baths of oil or molten metal, or even gas fired fur- 

 naces, since it is not necessary to use any part of the bridge as a heater. 



PERFORMANCE 



When it is used with platinum resistance furnaces at tempera- 

 tures below 1250° C, this regulator has been found to maintain the 



'' A recent experiment indicates that this interval may be decreased to 'A second by a 

 suitable design. 



