L. Page — A Century's Progress in Physics. 317 



ballistic rather than the static deflection recorded. 

 After the pressure produced by light from a carbon arc 

 had been measured, the intensity of the radiation was 

 determined with a bolometer. Preliminary experiments 

 indicated the existence of a pressure of the order 

 expected, and later more careful measurements showed 

 good quantitative agreement with theory. This pressure 

 had already found an important application in Lebedew's 

 explanation of the solar repulsion of comet's tails. 

 These tails are made up of enormous swarms of very 

 minute particles, and as the comet swings around the 

 sun they suffer a repulsion due to the pressure of the 

 intense solar radiation which counteracts the sun's gravi- 

 tational attraction. Hence the tail, instead of following 

 after the comet in its orbit, points in a direction away 

 from the sun. 



Some uncertainty existed as to whether a convection 

 current produces a magnetic field. A compass needle 

 is deflected by a current from a Daniell cell ; is the same 

 effect obtained when a conductor is charged electro- 

 statically and then whirled around the needle by means 

 of an insulating handle? The experimental difficulties 

 involved in settling this question are realized when the 

 enormous difference between the electrostatic and elec- 

 tromagnetic units of current is taken into consideration. 

 For a sphere one centimeter in radius, charged to a 

 potential of 20,000 volts, and revolving in a circle sixty 

 times a second, constitutes a current of little over a 

 millionth of an ampere. 



This problem was undertaken by Rowland (15, 30, 

 1878) in Helmholtz's laboratory at Berlin in 1876. A 

 hard rubber disk coated on both sides with gold was 

 charged and rotated about a vertical axis at a rate of 

 sixty revolutions a second. On reversing the sign of the 

 electrification on the disk, the astatic needle hung above 

 its center showed a deflection of over five millimeters. 

 The current was calculated in electrostatic units from the 

 charge on the disk and its rate of motion, and in electro- 

 magnetic units from the magnetic deflection. The ratio 

 of these two quantities gave fair agreement with its theo- 

 retical value, the velocity of light. 



Although the result of this experiment was confirmed 

 by Rowland and Hutchinson in 1889, Cremieu was con- 

 vinced by an investigation carried out at Paris in 1900 



