Scattering of Positive Electricity by Light. 439 



electrified system of conductors must be extremely well 

 insulated so that a feeble action of the light may not be 

 hidden by the loss of electricity not connected with the action 

 of the light. But apart from this, there is a disturbing cause 

 arising from the fact that each time the observer changes the 

 sign of the electric charge, there is a return current from the 

 insulating supports of that electricity which had passed to 

 them during the previous electric condition. 



Much less exposed to these sources of error is the arrange- 

 ment which Herr A. Eighi and we ourselves have often em- 

 ployed in photoelectric experiments, especially if it is necessary 

 to recognize feeble action. In this method the electrical 

 measuring apparatus, together with the conductor to be 

 illuminated, are at the commencement of the experiment at 

 zero potential, and the strength of the action of the light is 

 measured by the velocity with which the potential becomes 

 equal to that of a conductor kept at constant potential, which 

 stands opposite to the illuminated surface at a small distance 

 from it. 



The arrangement of the experiment was as follows : — The 

 ultra-violet light was furnished by the spark of a condenser, 

 which was connected with the poles of an induction-coil 

 actuated by 4 to 8 large Bunsen elements, the spark having 

 a maximum length of 18 centim. The current was broken 

 by means of a Wagner's hammer with platinum contacts, 

 and the spark of the condenser was taken between two alu- 

 minium wires at a distance of 2 millim. The galvanic battery, 

 the induction-coil, spark-space, and all the necessary con- 

 nexions were placed in the open air in front of the closed 

 window of the observing room. One of the panes of the 

 window was replaced by a plate of thin iron connected to 

 earth, which was provided with a circular opening, in which 

 was inserted a quartz lens of 50 millim. diameter. Since the 

 focus of this lens coincided with the spark, a parallel beam of 

 ultra-violet light was formed by the lens within the room, 

 whilst at the same time the electrostatic action of the induction- 

 coil and of the electrified air from the spark was shut off from 

 the room. Within the room, at a distance of about 25 centim. 

 from the window and at right angles to the beam of light, was 

 placed a piece of iron-wire gauze with a mesh of about 

 1 millim., and parallel to this, at a distance of 2 to 4 millim., 

 the insulated plate of the substance to be examined. From 

 this a wire went to the quadrant electrometer (sensitiveness, 

 1 volt = 23 divisions), whilst the wire gauze was charged 

 to a potential of about 525 volts by a battery of several 

 hundred Leclanch6 cells. According as the wire gauze was 



