4!£^,fad$i), 



THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



►•♦ 



Art. XXI. — Experiments with a Pendulum-electrometer, 

 illustrating measurements of static Electricity in Absolute 

 : by Alfred M. Mayer. 



Description of Apparatus. — A sphere, accurately turned 

 out of velvet cork to 1 cm. radius, with a smooth surface, was 

 heavily gilt. A staple made of tine wire was driven into this 

 sphere, and a very line filament prepared from a cocoon of the 

 silk-worm was passed through the staple and the ends of the 

 filament were attached, at a distance of 52 cms. apart, to a 

 support near the ceiling. In this bifilar suspension the effect 

 of torsion is eliminated, for the twist of one length of the 

 thread is opposed to that of the other. The vertical length of 

 the pendulum is 364 cms. or nearly 12 feet. 



A scale of millimeters, pasted on a slip of glass, is placed 

 behind the suspending threads and 34 cms. above the center of 

 the suspended sphere. A black band (not shown in figure) is 

 painted along the lower edge of this scale, so that, when the 

 line of sight is in the plane of the two threads of bifilar 

 suspension, we see a fine white line on this black ground. 

 This permits fine readings of deflections along the scale of 

 millimeters. 



The force pushing the pendulum from the vertical will be 



as the weight of the sphere and \ the weight of suspending 



thread into the sine of the angle of deflection. The weight of 



sphere and one half of thread is 990mgrms.. and the force of 



one dyne will give to the pendulum a deflection from the 



vertical of 3*4 mm. on the scale above the sphere, if we take 



g = 9802 cms. at Hoboken. 



Am. Jour. Sci.— Third Series, Vol. XXXIX, No. 234.— May, 1890. 

 22 



