DEPARTMENT OF TERRESTRIAL MAGNETISM. 273 



Further experiments concerning "magnet -photography." L. A. Bauer and W. F. G. 

 Swann. (Abstract of paper presented before the American Physical Society, 

 New York, February 15, 1917.) 



The paper describes a continuation of the work outhned in the abstract 

 on page 270. A large number of experiments have been performed, and it is 

 impracticable to give more than a general survey of the work. In all of the 

 experiments in which articles were exposed to the influence of a magnetic 

 field in the manner already described, a subsidiary plate was set up with a 

 similar set of articles, but beyond the influence of the magnetic field. In the 

 earlier experiments, this subsidiary plate was placed in an ordinary plate-box 

 at atmospheric pressure, the box being bound around with dark cloth ; but in 

 the later experiments it was placed in the same vacuum as the main plate, 

 and shielded from the magnet by an iron disk. All experiments except the 

 first two or three performed were carried out in total darkness, the photo- 

 graphic red light only being turned on after the articles had been mounted 

 under the bell-jar, and the latter had been bound aromid with black cloth. 



In order to test whether the effects observed could be attributed to radio- 

 active material, two experiments were set up with permanent magnets, but 

 in the case of one of them the bell-jar was washed out with a weak solution 

 of uranium nitrate and allowed to dry. After an exposure of 21 days, the 

 articles showed up equallj^ strongly on both plates. 



A plate was exposed for 6 days over an electromagnet, the articles being 

 pieces of wood, iron, copper, amber, and cork; also, in addition, a piece of 

 wood resting on a piece of lead which was in contact with the plate, and a piece 

 of lead resting on a piece of wood which, was in contact with the plate. Both 

 pieces of wood in contact with the plate showed up dark on a lighter ground, 

 the grain of the wood being very clear, and the cork showed up slightly darker 

 than the ground. The metals in contact with the plate all came out lighter 

 than the ground. In the case of the subsidiary plate which was set up at 

 atmospheric pressvu'e, only the amber showed up, and this appeared dark on a 

 light ground. 



Several experiments made with the articles slightly separated from the 

 plate showed that their influence fell off rapidly within a distance of 1 or 2 mm. 



The effect of resin in acting upon a photographic plate, especially when the 

 resin has been previously stimulated by light, is well known, and at once sug- 

 gests the assumption that the resin wood, cork, etc., produce a radiation of 

 some kind, or a gaseous emanation, the latter being produced either directly 

 or as a result of the radiation, and that this gas diffuses over the body of the 

 plate and darkens it. In this case the metal articles would simply act as 

 shields to the plate. Experiments made with stimulated resin and metallic 

 articles showed that distinct impressions of the articles could really be pro- 

 duced in this way without a magnetic field or a vacuum. At atmospheric 

 pressure the darkening of the plate falls off rapidly with the distance from 

 the resin, but on evacuating the space around the resin the "range" of the action 

 is increased and the impressions are much more uniform. 



A large number of experiments were made Avith stimulated resui, and it 

 appeared that the action was propagated roughly in a linear manner for a 

 distance of as much as a centimeter or more, and after traversing this distance 

 was still capable of passing through aluminum leaf. 



The apparent action of the magnetic field in the experiment with wood and 

 metal articles suggests that a similar action should be produced in the case 

 of the resin. If the action of the resin is ultimately attributable to the ejec- 

 tion of charged particles, the possibility of producing deviation by a magnetic 



