S6 Barus — Displacement Interferometer. 



be rough and the use of interference rings out of the question. 

 As the rings, however, may easily be obtained and used under 

 suitable conditions, and as the purpose of the present paper 

 is merely to test the method, the annoyances in question are 

 of less consequence. 



To obtain small potentials, a thin bare German silver wire 

 about 1 meter long was stretched and insulated on a board and 

 the ends supplied with a constant potential difference of one 

 volt. Two points of the wire, one or more centimeters apart, 

 were then used as a source of potential, the wires from these 

 points leading to a Mascart key. which suitably earths and 

 commutates the charged wires leading to the electrometer. A 

 thermo-couple might have been used ; but the long wire is 

 preferable because of its simplicity. 



In the first experiments made, the suspension was a bifilar 

 about 10 centimeters long (each strand of several silk fibers) and 

 about '5 to 1 millimeter apart. No doubt the torsional stress 

 of the fiber was here of an order commensurate with the bifilar 

 force. The needle, being damped in concentrated sulphuric 

 acid, moved very slowly and about two minutes were allowed 

 for each deflection. It was thus possible to follow the ellipses 

 on commutation, from one to the other extreme elongation, by 

 moving the micrometer screw proportionally to the displace- 

 ment of ellipses. As a rule, the ellipses were quite clear all 

 the way, showing that the adjustment for parallelism of mirrors 

 on the needle by the aid of sunlight is adequate. The potential 

 of the charge on the needle was of the order of 150 volts. 



In the first experiments, AiV '/A F= "57 centimeter was the 

 displacement on the micrometer per volt, so that A.JV = 10^* 

 centimeter corresponds to '000170 volt, or the sensitiveness is 

 about "000060 volt per vanishing interference ring. 



The instrument was nowimproved by inserting a longer bifilar 

 suspension, consisting of a single fiber of silk. The damper 

 and other details were retained. The potentials were tapped 

 from points of the long wire, respectively 5 and 10 centimeters 

 apart. Owing to the very large deflection, the ellipses were 

 not equally clear throughout the whole displacement from 

 elongation to elongation. 



The instrument behaved much better in the present case and 

 the sensitiveness had been much increased, particularly in the 

 later series of observations. Since AA r /A V was of the order of 

 2*9 centimeters per volt, 10 -4 centimeter corresponds to '000035 

 volt, i. e., to about '000010 volt per ring, so that under favor- 

 able circumstances a few millionths of a volt should be dis- 

 cernible. 



Brown University, 



Providence, E. I. 



