276 C. B. Rice — Reading Deflections of Galvanometers. 



Art. XL. — On a New Method for Reading Deflections of 

 Galvanometers ; by C. B. Rice. 



A METHOD of reading deflections of galvanometer needles 

 and analogous apparatus has been recently devised in this labo- 

 ratory by Professor C. S. Hastings, which, since it proves to 

 have considerable value, he has given me the opportunity to 

 describe. 



There are two types of sensitive galvanometers in general 

 use, namely, those in which the deflections of the suspended 

 system are determined by the motion of an image of a lumi- 

 nous source on a scale, and those in which the deflections are 

 measured by telescope and scale after the method first intro- 

 duced by Gauss and Weber. The Thomson galvanometers are 

 the familiar examples of the first, and the Wiedemann of 

 the second. 



Each of these has certain inconveniences. That of Thomson, 

 although an exceedingly sensitive galvanometer, requires a 

 darkened room and a lamp, with somewhat inaccurate determi- 

 nation of the angle of deflection. It is also rather too delicate 

 to put into the care of the untrained student. Wiedemann's 

 is much better suited for ordinary laboratory work. Here less 

 sensitiveness is compensated by much greater precision of angle 

 measure, while its construction renders it safe to put the instru- 

 ment into comparatively unskillful hands. A much wider 

 range of sensitiveness and of damping is readily secured in 

 this form than in Thomson's. It has, however, the inconveni- 

 ence of requiring the rather expensive accessory of a telescope 

 and scale and also of demanding a large amount of space in its 

 employment. These disadvantages are not of such great mo- 

 ment in most cases, perhaps, to forbid its use, and in our labora- 

 tory we found such instruments, well constructed of metal 

 throughout, thoroughly satisfactory until continuous disturb- 

 ances of magnetic field in the region were brought about by 

 the construction of an electric road. Experience showed that 

 the sensitiveness was thus reduced to about one-tenth its 

 former value. D' Arson val galvanometers of course eliminated 

 this difficulty, but they too have their inconveniences and 

 limitations. 



If the linear dimensions of a Wiedemann's galvanometer 

 were sufficiently reduced, say 7^-fold, the sensitiveness would be 

 increased, while the accuracy of determination of duration 

 would be decreased in the same ratio, since the apertures of 

 the optical apparatus are thus reduced. If this were carried 

 far enough, the aperture of the telescope would be reduced to 

 that of the pupil of the eye and might be dispensed with 



