290 II. A. Rowland et al. — Ratio of the Electromagnetic 



Description of Instruments. 



Electrometer. — This was a very fine instrument made partly 

 according to my design by Edelmann, of Munich. As first 

 made, it had many faults which were, however, corrected here. 

 It is on Thomson's guard ring principle with the movable 

 plate attached to the arm of a balance and capable of accurate 

 adjustment. The disc is 10 - 18 cm. diameter in an opening of 

 10-38 cm. and the guard plates about 33*0 cm. diameter. All 

 the surfaces are nickel plated and ground and polished to opti- 

 cal surfaces and capable of accurate adjustment so that the dis- 

 tance between the plates can be very accurately determined. 

 The balance is sensitive to a rag. or less and the exact position 

 of the beam is read by a hair moving before a scale and ob- 

 served by a lens in the manner of Sir "Wm. Thomson. The 

 instrument has been tested throughout its entire range by 

 varying the distances and weights to give the constant poten- 

 tial of a standard gauge, and found to give relative readings to 

 about 1 in 400 at least. It is constructed throughout in the 

 most elaborate and careful manner and the working parts are 

 enclosed in sheet brass to prevent exterior action. 



As the balance cannot be in equilibrium by combined 

 weights and electrostatic forces, it was found best to limit its 

 swing to a T \ mm. on each side of its normal position. The 

 mean of two readings of the distance, one to make the hair 

 jump up and the other down, constituted one reading of the 

 instrument. 



The adjustments of the plates parallel to each other and of 

 the movable plate in the plane of the guard ring could be 

 made to almost ^mm. 



The formula for the difference of potential of the two plates 

 is 



_ S7rd 2 wg 



A 



where cl is the distance of the plates, wg the absolute force on 

 the movable plate and A its corrected area. According to 

 Maxwell 



A=£*r| R 2 + R' 2 -(R' ,J -R 2 ). 



d-\-a J 



where R and R 7 are the radii of the disc and the opening for 

 it and a= - 221(R / — R). The last correction is only about 1 in 

 500, and hence we have, finally, 



r- A ,0002 I 



V=\l-221<y/wdl 1 + — ^— Y 



Standard Condenser. — This very accurate instrument was 

 made from my designs by Mr. Grunow, then of New York, 

 and consisted of one hollow ball, very accurately turned and 



