332 Mr. Otto Stuhlmann on a Difference in Photoelectric 



(fig. 1), so that their planes were perpendicular to the axes 

 of the cylinders. The plates had an area of one square centi- 

 metre, with parallel faces, although not ground optically 

 accurate. The corners were ground round to prevent leakage 



Fig. 1. 



Ecu-Tin 



S. QivccT»o« 

 4 \ o\ 



to the cylinders. They were mounted in brass clamps sup- 

 ported through the top of an earthed metal chamber, C, by 

 means of hard rubber bushings. A pointer, D, attached 

 to the supports, with suitably arranged stops allowed them to 

 be turned around their axes. The plates were connected 

 to the negative terminal of a 200 volt batterv. the other ter- 

 minal of which was grounded. Experiments showed that 

 this was sufficient to produce saturation. 



The brass cylinders, 5 cm. long and 2 5 cm. in diameter, 

 were provided with caps the opening through which was 

 1'5 cm. in diameter. This insured against the possibility of 

 having photoelectric ions drawn over into the adjoining 

 cylinder. The cylinders, resting upon sealing-wax supports, 

 were connected with a Dolezalek electrometer, giving about 

 900 divisions deflexion per volt, which was placed in an 

 adjoining metal case connected to earth. A narrow beam of 

 ultra-violet light, stopped down to 2 mm. by the opening at 

 S, was allowed to pass down the axes of the two cylinders 

 normally to the plates. An arc, with both rods made of soft 

 iron, running on 4 amp. direct current, was used as the source 

 of ultra-violet light. It was enclosed in a light-tight box 

 provided with a shutter, by which the beam of light passing 

 through the apparatus was controlled. The distance of the 

 arc from the plate B was 55 cm., and from the plate A 60 5 cm. 



