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PHYSICS. — Sensitivity and magnetic shielding tests of a Thomson gal- 

 vanometer for use in radiometry. W. W. Coblentz. Bureau 

 of Standards Scientific Paper No. 282 (Bull. Bur. Stds., 13: 423- 

 446). 1916. 



The present paper gives the results of an investigation of the force 

 exerted by various galvanometer coils when operated under standard 

 conditions. Some of the coils were wound according to theoretical 

 requirements, while others were wound empirically. Numerical 

 data are given relating to coils having various resistances. 



A simple coil is described, wound with a single size of wire (No. 

 28 B & S) which is as efficient as a compound coil wound upon the 

 same mandrel but in three sections of graded wire. 



A 9-ohm coil of graded wire is described which is very efficient and 

 is well adapted for use with the bismuth-silver thermopiles previously 

 described. 



A comparison is made of various astatic magnet systems, and data 

 are given showing the importance of using small mirrors, in order to 

 increase the sensitivity. 



Experiments in shielding the galvanometer from external magnetic 

 disturbances are described. Various shields are described consisting 

 of laminated cylinders made from transformer iron and solid cylindri- 

 cal shells cut from wrought iron gas pipe. By embedding the galvan- 

 ometer coils in blocks of Swedish iron which are surrounded by cylindri- 

 cal shells of transformer iron and of wrought iron, the effect of external 

 magnetic perturbations upon the astatic needle system is easily re- 

 duced to 1/2000 of its original value. This embedding of the coils 

 also reduces the air space ; the resultant elimination of convection cur- 

 rents greatly improves the steadiness of the needle system. 



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