248 THE ELECTKIC TELEGRAPH. 



Fig. 127 shows a plan of this arrangement, c c is a paper 

 scale divided into 1,000 equal parts, usually millimetres ; 

 a b, a telescope ; g g, the multiplier ; and m a magnetised 

 steel mirror, about one-eighth of an inch thick and 1 inch 

 diameter, polished on the side facing the telescope, and sus- 

 pended by a long fibre of unspun silk. When undisturbed 

 by the passage of a current through the coil, or from other 

 causes, the mirror takes the direction of magnetic north and 

 south, and reflects the central division of the scale into the 

 telescope ; but when a current passes through the coil the 

 mirror is deflected, making an angle, a, with the line n s, 

 and reflecting some division d of the scale into the telescope, 

 or that point in the line (d m) which makes with the line a o m 

 the angle 2 a. Within 5, the values of sines and tangents are, 

 within a very small fraction, equal to their angles ; so that, 

 when the angles do not exceed this limit, they may be 

 taken without further reduction as proportional to the cur- 

 rents producing them. With Weber's instrument this limit 

 is never exceeded : the length o c being 0'5 metre, and 

 o m, the distance of the scale from the mirror, usually more 

 than 5 metres. Besides, where the diameter of the mirror 

 is small in comparison with the diameter of the coil, we 

 have seen that the currents are proportional to the tangents 



of the angles a. With this instrument = tan. 2 a ; but 



as, for very small angles, we may put tan. 20 = 2 tan. a 

 without appreciable error, we can accept also the values of 

 o d as being proportional to the tangents, and therefore to 

 the intensities also. 



Fig. 128 shows a vertical section of such a galvanometer. 

 a a and d a! are two circular coils of fine copper wire insu- 

 lated with a covering of silk, forming the multiplier. In 

 some of these instruments the coils a a and d d are divided 

 into a number of coils of different lengths and gauges of 

 wire, terminating in a series of binding screws, b b, outside 

 the frame of the coil. The frame on which the wire is 

 wound is of vulcanite. The mirror m is suspended by a 

 fibre,/, from a little windlass, r, on the cap c of the vertical 



