632 



ELECTRICAL MEASUREMENTS 



BlondeFs suggestion for making the moving-coil galvanome- 

 ter available as an oscillograph is illustrated in Fig. 396. 



A single loop of a narrow and very thin conducting strip is 

 stretched over a frame in such a manner that the part between 

 the bridge pieces a and b is free. A very small and very light 

 mirror is cemented to the two sides of the loop midway between 

 the bridges. The loop is placed between the poles NS of a power- 

 ful electromagnet which is worked at high saturation. The 

 poles are large enough so that the free section of the loop is in a 

 practically uniform field. 



FIQ. 396. Showing principle of bifilar oscillograph. 



A current passing around the loop will cause one side to advance 

 while the other recedes; the mirror is thus turned around a 

 vertical axis. For the very small movements which are employed, 

 the deflections of the mirror are proportional to the current. 



The movable system is immersed in oil of such a viscosity 

 that, at the normal temperature of operation, the galvanometer 

 is dead beat. 



The material from which the strip is drawn should have a low 

 resistivity so that there will be little heating. This avoids 

 creeping of the spot of light due to the expansion of the strip 

 and reduces the energy consumed by the galvanometer. 



This form of instrument has the advantage that the inductance 

 is very small and is the one to which the most attention has been 

 given by designers in the United States and in England. 



Fig. 397 shows a group of oscillograph vibrators. The corre- 

 sponding galvanometers, complete, are shown in Fig. 398. 



In A, which is intended for high-tension work, a permanent 

 magnet is used. The disadvantage is the decreased sensitivity. 

 In B the galvanometer elements are thoroughly insulated from 



