Till-: MEASUREMENT OF Cl'KKl-:\T 



This simple form of instrument is unsatisfactory, for it is 

 not possible to attain an exceedingly high degree of sensitivity 

 owing to the loss of effective space near the needle, which results 

 from putting a comparatively large mirror within the coil. The 

 ion of the short suspension is also troublesome. More im- 

 portant still is the fact that the usable sensitivity is limited 

 by the variability of the local field, 

 due to outside magnetic dist urb- 

 ane. 



The galvanometer being of funda- 

 mental importance in all son 



rical testing, later experimenters 

 have devoted much time to the im- 

 provement of the details of the 

 Thomson type of instrument, the 

 ire being to attain a very high 

 degree of sensitivity, freedom from 

 outside magnetic disturbances, a 

 minimum but definite torsional con- 

 trol due to the suspension, propor- 

 tionality of scale reading to current, 

 and convenience of adjustment. 



A form of Thomson galvanometer 

 in which a simple magnetic system 

 is used, is shown in Fig. 5. The 

 needle is controlled and held in its 

 eiO position by the combined action 

 of the local field and the magnet M. 

 The amount of the control and thus the sensitivity, as well 

 as the position of the /ero reading of the instrument, may be 

 altered by raising or lowering M and turning it in azimuth. 



A simple instrument like this one is no longer useful, for 

 under conditions now well-nigh universal, the local magnetic 

 field is subject to such erratic variations both of magnitude and 

 direction that neither the zero nor the deflected readings can be 

 taken with certainty. This trouble increases as the sensitivity 

 of the galvanometer is raised by adjusting the magnet M. The 

 difficulties may be minimized in two ways: 



1. A truly astatic needle system may be used. 



. ~>. Simple Thomson 

 galvanoint t r. \ Cry susccp- 

 til.lt- to variations of tin- 

 local field. 



