370 



On a new Standard Sine- Galvanometer, 



coil uniformly is one to which mechanical appliances of great 

 precision can be readily applied. The wire can, for instance, 

 be laid on by means of a self-feeding lathe with a pitch which 

 is not only perfectly uniform, but which is also known with 

 minute accuracy from the pitch of the feed-screw. Any 

 slight irregularity which might remain can, if it be within 

 the sensibility of the instrument, be detected by moving the 

 needle along the axis by withdrawing the tube Tj and observ- 

 ing the change of sensibility. The great advantage of the 

 arrangement is the simplicity and possible accuracy of the 

 measurements and the great uniformity of the field all round 

 the central point. The resistance of the coil is, when thin 

 wire is used, somewhat higher for the same sensibility than 

 it is in the ordinary form, but for a standard instrument this 

 is of little importance. 



Referring now to the sketches, the tube T, which carries 

 the coil, is mounted on a platform P, furnished with three 

 levelling screws L, L, L, and can be turned round a vertical 

 axis V, sliding at the same time on two feet /, /. The 

 angle through which the coil is turned is measured by a 

 scale S. In the centre of the tube a small plane mirror m is 

 suspended, and at one end of the tube a short scale s, illu- 

 minated by means of an inclined mirror or prism, which 

 receives light through a small hole h, is fixed, and immedi- 

 ately above it a plane mirror M. The light from the scale s 

 is reflected from the suspended mirror to the fixed mirror M, 

 and then through the telescope t fixed in the end of the tube 

 Tj. The scale s here shown may be replaced by a narrow 

 slit, or a round hole with a wire across it, and the telescope 

 by a sheet of obscure glass. A lens placed in front of the 

 opening may then be used to focus an image of the slit or wire 



