; 



Protective or Circuit Breaking Device. 271 



was not only to study the time functions in circuit breakers 

 now in use, but also to devise additional protective methods. 



In the three devices described above the operating principle 

 is that of magnetism. 



With the exception of one or two devices where the expan- 

 sion of metallic rods is used for completing a tripping circuit, 

 no work, having heat for its operating principle, seems to be 

 recorded. Attacking the problem from this point of view, the 

 principle of the Nernst lamp was first used. The amount of 

 current in the glower is some function of its temperature. If 

 then the line current through the heater heats the glower, 

 and at a certain temperature the glower allows enough 

 current to pass through its shunt from the line to energize a 

 tripping magnet, the system furnishes a good inverse time 

 element. For the higher the current in the heater, the shorter 

 will be the time necessary to heat the glower to the required 

 temperature. 



An old style of JSfernst heater was obtained and the glower 

 placed on the inside. Even with the glower in this position 

 it took from \\ to 2 minutes for \ of an ampere to heat 

 the glower to the temperature required for operating the 

 tripping magnet. The glower and the magnet in series were 

 placed in shunt with the heater. The time for heating the 

 glower to the operating point was too long, and the current 

 used too high, to give satisfaction. 



After an extensive search it was found that fused lead chro- 

 mate has properties similar to those of the Nernst glower. In 

 the same position as the glower, it took less time and current 

 to operate the tripping magnet. The behavior of the chromate 

 seemed very promising and hence it was thought profitable to 

 plot a curve connecting its resistance with temperature. 



The accompanying diagram (fig. 1) shows the connections of 

 the apparatus used for obtaining the data. 



HI is the electric furnace in which the chromate was heated. 

 The temperature was determined by a compensating lead type 

 of platinum thermometer. The thermometer was connected 

 to the ordinary form of a slide wire resistance bridge as shown 

 in the diagram. <3r 2 is a Leeds and Northrup testing galvanom- 

 eter ; S 2 is a variable standard resistance chosen very nearly 

 equal to the platinum, so that the balance was obtained near 

 the center of the bridge. P 1 P 2 G t C 3 are the terminals of the 

 thermometer. The leads from these terminals to the bridge 

 were specially constructed for this purpose, with the aim of 

 making the bridge very symmetrical and more accurate. 



The platinum of the thermometer was incased in a porcelain 

 tube, and to have as nearly as possible a correct temperature 

 of the chromate, this was mounted in a similar way. A por- 

 celain tube F of about the same size as that of the thermome- 



