300 Mr. I. 0. Griffith on the Electricity set free 



As the plate AA acquires a positive charge, the plates d 

 are brought from zero to negative potentials so as to keep 

 the electrometer-needle approximately at rest. At the end 

 of the experiment the potential V of the plates cl required to 

 bring the needle exactly to zero is found. The quantity of 

 electricity given out is measured by VC, where C is the 

 capacity of the plates in use. 



The potential of the plates is adjusted by a potentiometer 

 method. A set of 50 equal resistances of 20 ohms each were 

 arranged in series, and by means of a sliding contact it was 

 possible to connect the plates with any one of the junctions 

 of the 20-ohm coils. The two terminals of the 1000-ohm 

 resistance were joined to a battery of known E.M.F. E, the 

 positive pole of which was connected to earth. 



The value of V can now be estimated accurately by ob- 

 serving the small deflexions on opposite sides of the zero 

 when the plates are in connexion with two consecutive 

 junctions of the resistances. Quantities of electricity can 

 thus be measured accurately over a large range, as C and E 

 can be easily altered. Further, the electrometer is only 

 used as a detector, so that its capacity need not be determined 

 and all errors due to unequal values of the readings at 

 different parts of the scale are eliminated. 



The electrometer used was of the Dolezalek pattern and 

 gave a deflexion of about 3000 divisions for 1 volt. 



The source of ultra-violet light was a spark-gap in a circuit 

 in which a leyden-jar discharge took place, the spark passing 

 between aluminium terminals in air. The leyden-jar was 

 charged from the secondary of a Ruhmkorff coil. All the 

 apparatus in connexion with the spark was enclosed in a 

 metal-lined box permanently earthed in order to prevent 

 induction effects on the insulated conductors. With a proper 

 adjustment of the current in the primary circuit of the 

 induction-coil and a proper rounding of the terminals, the 

 spark in air was found to be very constant. 



Method of performing an Experiment. 



Various intensities of light of known relative values can 

 be obtained by varying the distance between the spark and 

 the detector. If we assume the intensity of the light falling 

 on the detector to vary inversely as the square of the distance 

 from the spark-gap, it will be necessary to ensure that the 

 same quantity of light is absorbed for the different distances. 



