450 PROCEEDINGS OF THE AMERICAN ACADEMY 



part of the metallic shield, covering and perfectly protecting the needle 

 and quadrants from external electrical influences and also air currents. 

 Circular brass windows 7 cm. in diameter, encased in brass and in- 

 serted in three sides of the case, allow inspection from without, of the 

 needle, and also the passage of the beam of light to and from the 

 mirror. 



The bottom of the compartment consists of a brass plate of about 

 5 mm. thickness. From this plate rise, at alternate corners, two 

 brass rods of 23 cm. height, supporting a cross-beam 20 cm. in 

 length and 5 mm. in diameter. Fastened by a screw to this beam 

 is a cross piece of 3.5 cm. length, supporting two suspending rollers. 

 One roller has a screw movement to or from a central point exactly 

 over the centre of suspension. The end of the roller is enlarged and 

 grooved. One end of the suspending fibre is fastened to the roller by 

 insertion through a small eye-hole in the shaft, and then made to pass 

 in the enlarged groove. The other roller consists of a like brass shaft 

 with an enlarged groove, in which the other end of the suspension 

 fibre runs. This roller has not, however, a screw movement, and 

 turning the screw head in this case simply raises or lowers the needle 

 without change of position of the points of suspension. The whole 

 suspension may be raised or lowered by movement of the screw- 

 head attached to the supporting cross-beam. The plane of suspen- 

 sion may be altered by movement of the cross-bar, which is pivoted 

 on the end of the screw passing through the cross-beam. This 

 suspension is much simpler, equally sensitive with the best arrange- 

 ments in other instruments, and in case of accident easily repaired. 

 The length of the suspension is about 9 cm. Only one long fibre 

 is employed. The platinum wire carrying the needle is hooked at 

 its upper extremity, and by this means attached to the fibre. The 

 weight of the needle is sulfieient to insure a symmetrical suspen- 

 sion, without extra adjustment. If two separate fibres are used, 

 and attached to a small cross-piece on the platinum wire, it will 

 be necessary to test the symmetry of the suspension. The single 

 fibre, however, allows a symmetrical suspension and with the arrange- 

 ments employed allows easy and accurate adjustment. In the instru- 

 ment constructed there is no error of position of the zero point. After 

 the greatest deflection when short-circuited, the position of the spot 

 of light on the ground-glass scale is exactly that of its initial position. 

 Six months' cgnstant use of the instrument has not necessitated the 

 use of any correction for the position of the zero. 



The needle is made of aluminium about 10 cm. in length, and at 



