Intelligence and Miscellaneous Articles. 427 



ON THE LEAKAGE OF NEGATIVE ELECTRICITY CAUSED BY SUN 

 AND DAYLIGHT. BY T. ELSTER AND H. GEITEL. 



In reference to the theory of atmospheric electricity propounded 

 by Arrhenius*, we have recently made a series of experiments 

 to ascertain whether sun- and daylight have the property of 

 gradually withdrawing the charge from negatively electrified bodies. 

 M. Hoorf alone has established such an action, while all other 

 observers, so far as we know, have not been able to discover 

 any. We have therefore been greatly surprised to find that not 

 only sun- but also ordinary diffused daylight can under suitable 

 conditions rapidly discharge a negatively electrified body. 



A zinc dish, 20 centim. in diameter, is exposed in the open on 

 an insulating support in such a manner that it is not acted on by 

 negative electricity, and is put in conducting communication with 

 a quadrant-electrometer or an Exner's electroscope, and further is 

 so arranged that the dish can be put in the dark or light at pleasure. 

 The following phenomena can then be observed, which, it is true, are 

 already known from experiments on ultra-violet light. The dry dish 

 polished with emery completely loses a negative charge of 300 volts 

 in 60 seconds : an equally high positive charge is retained. The 

 loss of the negative charge ceases as soon as the dish is put in an 

 entirely dark room ; it is considerably enfeebled if the sun's rays 

 pass previously through a glass plate. A decided collapse of the 

 leaves of the electroscope takes place when the dish is merely 

 exposed to the blue light of the sky. 



If the dish is filled with hot or cold water the action is com- 

 pletely extinguished ; a moist cloth stretched over it acts in like 

 manner. 



By being illuminated the finely polished plate acquired a sponta- 

 neous charge of + 2-5 volts, which by blowning on the plate could 

 be still further increased. 



The experiments are much simpler when the metals to be 

 illumined are directly fixed in the form of wire to the knob of an 

 Exner's electroscope. If freshly polished wires are used — alumi- 

 nium, magnesium, or zinc — a permanent negative electrification in 

 the sunlight in the open is not at all possible. It is completely 

 discharged in less than five seconds. Magnesium and aluminium 

 wires act here more energetically than zinc ones. There is a per- 

 ceptible collapse of the leaves when the former are used, even with 

 the action of diffused evening light. 



It is also interesting to note that freshly polished wires of the 

 metals in question act as if an ignited body were attached to the 

 electroscope. If an electroscope so arranged is taken to an open 

 field, the leaves diverge with the use of freshly polished wires with 

 positive electricity, arising from the influence of the electricity of 

 the air. 



In all these cases an abnormal diffusion of positive electricity 

 could not be observed. 



* Meteorol. Zeitschrift, v. p. 297 (1888). 

 t Rep. der Phys. xxv. p. 105 (1889). 



