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LXIV. Description of a very Sensitive Form of Thomson 

 Galvanometer, and some Methods of Galvanometer Con- 

 struction. By F. L. 0. Wadsworth*. 

 [Plate XIV.] 



AS considerable interest has of late been manifested, parti- 

 cularly in Germany, in the construction of very sensitive 

 galvanometers, a brief description of one, which has recently 

 been constructed for the bolometric work of the Observatory, 

 may not be out of place in connexion with my last article 

 (p. 482). This galvanometer in question is of the Thomson 

 type of construction with four coils wound under my instruc- 

 tions by Messrs. Elliot, Bros. 



Fig. 1, Plate XIV., is a section through one of ihe coils 

 showing the contour of the individual sections, of which there 

 are five, of about 4 ohms each, in each coil. The size of 

 wire, number of turns, and resistance in each section of one 

 of the coils, which is typical of all of them, is given in the 

 following table. 



Table I. — Coil marked A. 



Section. 



Size wire. 



No. of 

 turns. 



Resistance 

 (at 20° C). 



No. I. (inner) 



00065"(37 B.W.G 



)• 



255 



4-48 true ohms. 



No. II. 



0-012" (30 



) 



410 



421 „ „ 



No. Ill 



0-021" (25 



) 



640 



413 „ „ 



No. IV. 



0-025" (23 



) 



551 



4-25 „ „ 



No. V. (outer) 



0-030" (22 



) 



540 



4-28 „ „ 



Total number of turns 

 Total resistance 



2396. 

 21*35 ohms. 



The radius of the inner coil is 2 millim., and that of the 

 outer coil 50 millim., and the depth of the coil was about 

 40 millim. As will be seen from the above table, the 

 diameter of the wire increases somewhat less rapidly than the 

 mean radius of the section, as required by Maxwell's theory, 

 the thickness of the insulating covering, while not constant, 

 being proportionally thicker for the finer than for the larger 

 sizesf. The total number of turns in the four coils is 9593, 

 and the total resistance (in series) 86 ohms at 20° C. 



* Communicated by the Author. 



t Maxwell, Elec. and Magnetism, vol. ii. p. 363. 



Phil. Mag. S. 5. Vol. 38. No. 235. Dec. 1894. 2 P 



