the circuit immediately after extinguishing the light. If no changes had occurred, the galvanometer would indicate a deflexion corresponding to the former resistance of a cell in darkness; but, if a change had occurred, the galvanometer would indicate a larger deflexion, and would gradually settle back to its original position. As a preliminary experiment bore out these conjectures so admirably, an exact determination was undertaken. A new cell was made and, upon closed circuit, was exposed to light for 30 seconds. The galvanometer-needle, upon extinction of the light, moved gradually towards its original position and, at definite time-intervals, readings of the galvanometer were taken. Then, with the circuit open, the cell was again exposed for 30 seconds, and, at the same intervals as before (after turning off the light), the circuit was closed for an instant, just long enough for the galvanometer needle to come to rest so as to obtain a reading. An inspection of the following Table II. and fig. 4 shows that, at corresponding intervals of time, the cell had the same resistance whether a current had been flowing during the time of exposure or not. On account of the first rapid drop in the curve, and also due to the comparatively slow motion of the galvanometerneedle, it was not possible to obtain readings on the steeper parts of the curve. However, the excellent agreement between the two parts of this experiment would indicate that a selenium cell, when exposed to light, experiences the same changes in resistance whether a current is flowing or not. In Bidwell's theory the phenomenon is explained by assuming that light facilitates the molecular re-arrangement in the surface-layer of a selenide "through which an electric current is passing." The experiments just described show that apparently the changes in resistance occur independently of the flow of current. Furthermore, according to Bidwell's theory it is most difficult to account for the role played by the large excess of free selenium, whose presence is absolutely necessary to the development of sensibility in a cell. Granting that in a selenium cell most, if not all, of the conduction is electrolytic in character, due to the presence of a selenide, it follows that there is an actual motion of the components of the selenide towards the electrodes of the cell. Any cause which will increase the velocity of these components will decrease the resistance of the cell. Selenium is known to exist in at least four allotropic modifications*, the metallic or crystalline form being represented in the selenium cell. As it is an established fact that light affects the character of certain crystalline compounds, it is not unreasonable to suppose that light, in falling upon selenium, might also change its crystalline character, and that this new modification might offer less resistance to the components of the selenide as they wander towards the electrodes, thereby producing indirectly an increase in their velocities, which is equivalent to a decrease in the resistance of the cell. This view gains in plausibility if, with Bidwell, we think of the particles of selenide as being packed in between the particles of selenium. Assuming that this new modification of selenium is stable only in light, it would revert to its original condition when light is cut off, the change taking place more rapidly at first and more slowly afterwards (comparable perhaps to the molecular changes in soft iron when the magnetizing force has been withdrawn). This would decrease the velocity of the components of the selenide, which would mean eventually bringing the resistance of the cell back to its original value. An explanation of this character has the following advantages: 1. It ascribes, with Bidwell, the electrical conduction to the selenide. 2. It assigns a definite role to the free selenium. 3. It accounts for the fact that light produces changes in the resistance of a cell whether a current be flowing or not. 4. It affords a possible explanation of the fact that the position of maximum sensibility is independent of the metal in the selenide. Of course, quite a number of other explanations are conceivable. It appears to me, however, that, as soon as the action of light directly upon the selenide is involved, some further hypotheses must be made to account for the fact that the position of maximum sensibility is independent of the nature of the selenide. The reason is at once apparent if the action initially be ascribed to the selenium itself, which is present in such abundance in all cases. * A. P. Saunders, Journal of Phys. Chemistry, vol. iv. p. 423. Résumé. 1. The sensibility of selenium cells, containing highly purified selenium mixed with various metallic selenides, was examined in different parts of the spectrum, and it was found that the position of the maximum, near 0.7 μ, remained fixed. It was concluded that the position of the maximum was not governed by the metal in the selenide, but probably by the free selenium itself. 2. It has been shown that a selenium cell, taken from darkness into light and again returned to darkness, undergoes changes in resistance whether an electric current flows or does not flow through the cell. 3. A suggestion as to the possible action of a selenium cell has been made. It is supposed that light affects the selenium directly rather than the selenide. This explanation, expressed of necessity in rather indefinite terms, gives promise of accounting for certain phenomena, the explanation of which fails at the hands of the existing theory. This investigation has been carried on during the past year under the supervision of Professor C. E. Mendenhall, to whom I wish to express my warmest thanks for his many valuable suggestions, and for the kindly interest he has taken in the progress of the work. Physical Laboratory, University of Wisconsin. V. The Bending of Magnetometer IN Deflexion-Bars. By (From the National N May 1901 I communicated to the Society a paper † making various applications of Elastic Solid Equations to Metrology. Amongst the examples treated was the bending of magnetometer deflexion-bars. As explained (.c. pp. 613-615), the deflecting magnet is carried by the deflexion-bar at an appreciable height above the C. G. of the cross-section, and the bending of the bar when in use, under its own weight and that of the magnet with its carriage, results in an increase of the distance between the deflecting and deflected magnets. To keep the instrument properly level, there ought to be a counterpoise on the other arm of the deflexion-bar, at the same distance as the deflecting magnet from the centre. In the absence of such a counterpoise, supposing the instrument originally level, the weight Communicated by the Physical Society: read October 23, 1903. + Phil. Mag. [6] vol. ii. pp. 532-558 & 594-616. of the magnet and carriage causes a slight tilting. In consequence of this, the point of suspension of the deflected magnet moves towards the deflecting magnet, thus reducing the horizontal distance between them. This compensating effect increases with the length of the suspension of the deflected magnet; it also depends on the pattern and massiveness of the magnetometer. The tilting is objectionable for several reasons, and suitable counterpoises exist in some instruments. In others, however, there is no regular counterpoise, the only equivalent being a thermometer, usually considerably lighter than the magnet and carriage, whose position varies according to the ideas of the observer, who may even put it on the same arm as the magnet. I have investigated the tilting effect in one or two cases, but refer to it at present only to put observers on their guard. If it exists, but is overlooked, the corrections deduced from a pure bending experiment are not strictly applicable. Since the publication of my first paper on the subject measurements of the bending effect have been made on over twenty magnetometers at the National Physical Laboratory; and it is now the regular practice to take the effect into account in framing the certificates, as these record lengths to 0.001 cm. The earlier measurements were made by Mr. F. E. Smith, the bar being symmetrically loaded, and the consequent increase in distance between two carried points being read by microscopes. After regular work was commenced at Bushy House, the measuring-apparatus was transferred there, and for some time magnetometers were sent over from Kew to Bushy, so that the measurements might be made by Mr. Smith on the bars supported as in use. The transport of heavy magnetometers being troublesome, and the degree of accuracy absolutely necessary being comparatively small, I devised the following method which enables the measurements to be taken at Kew. As actually carried out, the method makes no claim to the highest precision; but it is, I believe, novel and capable of development, and it could easily be applied by the owner of any of the older magnetometers which have been verified at Kew, the great majority of which are probably situated where there is no ready access to physical laboratories. The magnetometer is set up exactly as in an ordinary deflexion experiment, with the deflecting magnet on its carriage at any convenient position on the deflexion-bar, and equal weights are hung up symmetrically, one on each arm. The consequent increase of distance between the two magnets 7 |
