278 C. B. liice — Heading Deflections of GalvoMometers. 



point, preferably of some dark material with a white back- 

 ground. Directly under the lens is a small scale. The image 

 of this scale as formed by the mirror is seen superimposed 

 upon the image of the fixed point as seen through the aperture 

 in the mirror, and any change in the angular position of the 

 mirror causes a relative shifting of these two images, which, 

 under proper adjustment, are in the same plane and are, there- 

 fore, perfectly free from errors of parallax. The use of the 

 lens serves the double purpose of reducing the linear dimen- 

 sions of the apparatus and of giving such precision to the 

 ocular accommodation that the edges of the hole in the mag- 

 net as well as the fixed image of the scale due to the glass win- 

 dows are so far out of focus as to be quite invisible. When 

 the lens, point and small scale are once adjusted the instru- 

 ment is ready for use at any time with no further attention. 

 If the support be moderately level no leveling screws are 

 necessary. 



With this galvanometer a potential of YoT,o"iro" ^^ ^ ^^^^ ^^ 

 readily detected by repeated closing of the circuit — a sensitive- 

 ness which is more than sufiicient for a large part of the ele- 

 mentary work of the laboratory. 



The simplicity and cheapness of this type of galvanometer 

 make it very desirable, while its lack of delicacy is of consid- 

 erable moment where the care of it falls upon the novice. 



There seems to be no reason why this method for reading 

 deflections should not be used with all other galvanometers as 

 a substitute for the telescope and long scale. The angular 

 measures are read with great accuracy and with much greater 

 precision than with the lamp and scale of Thomson's method. 

 The small scale used in this connection should be divided into 

 half millimeters, for the reading is made with clearness to 

 tenths of such an interval. 



The instrument figured is one of those constructed at this 

 laboratory and must be looked upon simply as an example of a 

 convenient, moderately sensitive, but widely useful form. The 

 two coils are separated by a copper ring about 3°^°^ in thickness 

 which produces a certain damping, although the galvanometer 

 serves very well for ballistic use. JN^otwithstanding the mod- 

 erate damping, the actual time required to secure a succession of 

 readings is certainly not greater than is found necessary in 

 using a Wiedemann aperiodic galvanometer, because the period 

 is so greatly reduced. In an astatic system with long period 

 more effective damping would doubtless be desirable, but in 

 such a case there would be no difficulty in employing Thom- 

 son's ingenious air damping. 



It is obvious that a needle constructed of small steel mag- 

 nets cemented to silvered glass, the silver being removed from 

 a small portion, might replace, and in some cases advanta- 

 geously, our steel mirror. 



Physical Laboratory, Sheffield Scientific School. 



