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XLVIII. On the Application of the Clark Cell to the Construc- 

 tion of a Standard Galvanometer. By Professor Richard 

 Threlfall, M.A.* 



[Plate XIV. figs. 1 & 2.] 



IN the instrument which forms the subject of this paper the 

 experience obtained with Clark's cell is utilized to obtain 

 a simple means of standardizing a working current-measurer. 

 The construction of the instrument itself will be readily under- 

 stood from the accompanying figures. Its chief features are : — 



(1) The arrangements which have been made for the sup- 

 port of the controlling magnet and for its adjustment : this 

 latter can be readily carried out without disturbing the sus- 

 pension. 



(2) The damping of the needle by means of a thin copper 

 cylinder attached to a bit of fine wire and dipping in clove-oil. 



(3) The mechanical arrangements of this part of the appa- 

 ratus, allowing of the easy suspending of the mirror and 

 adjustment of the cylinder in the oil. 



(4) The arrangements for the testing of the galvanometer 

 by means of the Clark cell. 



(5) The curving of the scale, so as to obtain direct tangent- 

 readings from a scale of equal parts. 



The single coil of the instrument consisted of a rectangular 

 section of winding of 200 turns of No. 22 B.W.G. copper 

 wire. Resistance 2*02 ohms at 16° C. 



This coil had an axial dimension of 1'3 centim., a radial 

 dimension of 1*3 centim., and the radius of the inner layer of 

 winding was 3*7 centim. The coil was supported so as to be 

 capable of sliding backwards and forwards with respect to 

 the suspended parts, keeping parallel with itself to a con- 

 siderable degree of accuracy. The slide was of carefully 

 crossed wood, the moving portion being kept in its position 

 in the grooves by means of half carriage-springs at each end. 

 A somewhat similar arrangement is adopted in the sliding 

 wooden parts of the Kew magnetometer. 



There are three marked positions of the coil with respect 

 to the suspended needles. 



Though the mirror is rather large the magnets are small, 

 in the ordinary sense, i. e. about a quarter of an inch long. 

 A calculation was made by Mr. Adair on the law of deflexion 

 of the magnet by the coil at the three distances and up to 

 deflexions of about 15°. The method adopted in this calcula- 



* Communicated by the Physical Society : read March 28, 1889. 



