52 



ALTERNATING CURRENTS 



The current 



/ = K'VD (27) 



where K' = l/VK 



As the deflections are proportional to the square of the current, 

 the instrument gives effective values when equation (27) is 

 applied. It therefore reads correctly for both alternating and 

 direct currents. When direct current is used, it is advisable to 

 reverse the direction of the current and average the set of read- 

 ings. This eliminates the effect of stray fields. 



This type of instrument is difficult to adjust and to manipulate, 

 especially when the current fluctuates. It is not direct reading 

 and because of its construction is adapted to laboratory work 

 only. 



If small wire be substituted for the coarse wire and an exten- 

 sion coil be connected in series, the instrument can be used 

 as a voltmeter. 



28. The Indicating Electro -dynamometer. As it is neither 

 portable nor direct reading, the Siemens dynamometer itself is 



not adapted to portable and to 

 switchboard instruments. 

 However, many types of port- 

 able and switchboard instru- 

 ments operate on the Siemens 

 dynamometer principle. The 

 general construction of a port- 

 able type of electro-dyna- 

 Tmometer instrument is shown 

 in Fig. 49. 



Two fixed coils, FF r , are so 

 connected that their magnetic 

 fields act in conjunction. These 

 coils may be considered as being 

 two parts of a single coil, opened 

 in the middle to allow the spin- 

 dle of the moving coil to pass 

 through. 



M is a movable coil mounted on a vertical spindle. There is a 

 hardened steel pivot at each end of the spindle, which turns in 

 jewelled bearings. Two spiral springs similar to those used in 



FIG. 49. Principle of the electro- 

 dynamometer. 



