EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 113 



probably attributable to air-currents, convection currents due to temperature 

 differences and pendulum motion of the needle resulting from tremors. Steel 

 must always be excluded from the framework of the needle. 



The sensitiveness as is otherwise known, theoretically, does not in any case 

 increase with the potential of the needle, but passes through a maximum (in 

 the above designs) usually at about 150 volts. This is the case both with 

 the sharp-edged and the cylindrically-faced biplane needles. The directing 

 force in the case of such needles is essentially electric; i.e., they are set in a 

 position of equilibrium relatively to the quadrants by electric stress large in 

 comparison with the torque of the bifilar. As soon as these forces increase at 

 the same rate as the potential of the needle, the further increase of the latter 

 is no longer serviceable. Hence the biplane needle, set in relatively wide 

 quadrants, was found to offer the best conditions of sensitiveness, and it is 

 in the case of needles and quadrants of this design that the best results were 

 obtained. In other words, the sensitiveness also passes through a maximum 

 as the mean distance between the outside contours of the needle and the inside 

 contours of the quadrants increases. 



After preliminary experiments, the optics of the instrument offered no 

 serious difficulty. It is merely necessary to follow the reflected light by placing 

 white screens behind each mirror in the direction of the impinging rays. Since 

 the rays are reflected at the grating, the returning ray also necessarily passes 

 through the grating, and this part of the adjustment is therefore automatic. 

 With a copper-framed needle, the water damper will probably not be essential, 

 in which case the discrepancies due to surface viscosity will also disappear. 



