ELECTRICAL PROPERTIES OP FLUORESCENT SOLUTIONS. 1 65 



The tube, after being charged with anthracene, was exhausted to a 

 pressure of a few tenths of a millimeter, sealed off, and heated electrically. 

 Light from a carbon arc A (Fig. 162) was converged by the lens L, and could 

 be passed into the furnace through a mica window or could be cut off by the 

 screen 5. When the light was on, a bright cone of violet fluorescence could 

 be seen in the center of the tube. 



The outer electrode was charged from a storage battery B, and the rate 

 at which the inner electrode acquired a charge was measured by means of a 

 sensitive Dolezalek electrometer, which was inclosed in a wire cage to pro- 

 tect it from the electrostatic disturbances. The end of the anthracene tube 

 projected through a hole in this cage, making the protection of the inner 

 electrode practically perfect. 



OBSERVATIONS AND RESULTS. 



The electrometer needle was charged to a constant potential (in most of 

 the work this was 60 volts), one pair of quadrants was grounded, and the 

 other pair connected to the wire electrode. This pair could be grounded 

 or insulated at will by means of the key K . When K was raised the motion 

 of the needle was noted by observing on a ground-glass scale the image of a 

 lamp filament formed by the small concave electrometer mirror. 



If V g be the potential of the charging quadrants ; K the deflection when 

 V g is 1 volt; C the capacity of the charging system; Q the charge; D the 

 deflection; and /, the current; then 



D = kXV g and Q = CV q 

 I = dQ/dt = C X dV/dt = k X C X dD/dt 



on the assumptions that k 1 is the same for the moving needle as for the 

 needle at rest and that C does not change with the position of the needle. 

 Such assumptions are allowable for slow motion and small deflections. 



C was determined by dividing the charge with a cylindrical condenser of 

 30 cm. capacity, and was found to be 45 cm. or 5X 10 -5 microfarads. 



A charging rate of 1 division per second could easily be observed if 

 definite. This would mean a current 1.3 Xio~ 13 amperes. The insulation 

 was such that the rate of leak when charged to 50 divisions deflection was 

 about 10 divisions per minute, or one-sixth the rate mentioned as easily 

 measurable for charging. 



At the low pressure used, anthracene vaporizes sufficiently to show fluores- 

 cence between 200 and 250 C. "90 per cent sublimed" anthracene was 

 used without attempt at purification, as it was easily obtainable and the 

 presence of impurities in the material does not seem to affect the fluores- 

 cence of the vapor. No attempt was made to shield the electrodes from 

 the light scattered by the end of the tube, nor to use monochromatic light. 



In liquids there is a photo-electric effect, as has already been shown in 

 this chapter, dependent upon which of the electrodes is illuminated. The 

 possibility of the existence of such an effect in the vapor was not here 

 considered. 



In most of the work the outer electrode was charged to 120 volts from 

 an ordinary storage battery. K was raised and the rate of motion noted 

 for the unilluminated tube. This was repeated with the light on. 



'When the potential of the needle was 60 volts the value of k was found to be 375 mm. on a scale 

 one meter distant. 



