Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
Quantitative measurement is more difficult, but Prof. 
Mayer has lately used a pendulum electroscope in a form that 
makes a most sensitive electro¬ 
meter. He suspends a gilt 
pith-ball 1 cm. radius (built up 
of small pieces cemented) a 
distance of 364 cm. from the 
ceiling, by two silk fibres 
fastened 52 cm. apart and 
meeting at the ball, behind 
which a scale is arranged. A 
brass ball of the same size is 
mounted on a glass rod, var¬ 
nished while warm with 
paraffin wax. A force of only 
one dyne acting on the sus¬ 
pended ball deflects it 13*3 
mm. ; and charging both balls 
in contact so as to give the 
same charge to each, the 
charge c on either was found 
in absolute electrostatic units 
by the formula 
C = D 
where d is the distance in 
cm. between the centres of 
the balls, and cl the deflection 
from the vertical in cm. 
When d is over 5j cm. the 
law of inverse squares was 
demonstrated within 1 per 
cent ; and by using a proof- 
plane, Coulomb’s law of distribution on the pay 
drical body was verified with close approxima/ioh> Shell 
LJ # /


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