CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, 



HARVARD COLLEGE. 



A SIMPLE DEVICE FOR MEASURING THE DEFLECTIONS 

 OF A MIRROR GALVANOMETER. 



By B. Osgood Peikce. 



Presented April 11, 1906. Received May 25, 1906. 



For a good many years we have used in the Jefferson Laboratory, 

 to determine the deflections of mirror galvanometers and other similar 

 instruments, a simple form of tubeless telescope, which, at first in- 

 tended for students only, has proved in its way very convenient, and 

 has been adopted for some standard fixed instruments, because it is 

 not easily put out of adjustment after it has once been properly set up, 

 and because the scale image is so large and clear that a prolonged use 

 of the device does not involve the eye strain which most reading 

 telescopes would cause. 



B 



Tm 



HL 



M 



Figure A. 



In front of the plane mirror (M) of the galvanometer, instead of the 

 usual cover glass, is placed (Figure A) a convex lens (A) of focal 

 length equal to the desired scale distance. This lens need not be 

 achromatic ; a spectacle lens is quite good enough. At a distance in 

 front of A equal to its focal length is placed a horizontal scale (S) 

 mounted on a thin strip of wood at least twice as wide as the scale 

 itself Through the middle of this strip, above the scale, is bored 

 a round hole (H) rather more than 20 millimeters in diameter ; just 

 behind the scale is stretched a fine vertical wire or silk fibre (W) to 

 serve as a cross- hair. Behind H and at a distance suited to its focal 



