Abstract
LONDON. Physical Society, April 25.—Prof. S. P. Thompson, president, in the chair.—Dr. Dawsori Turner exhibited and de; scribed a mechanical break for induction-coils. The use of induction-coils in the production of Rontgen rays and in wireless telegraphy has made the construction of a suitable break a matter of importance. The ordinary break is unsuitable because o the wearing away at the point of contact, and there are objections to the use of mercurial breaks. The portable mechanical break which was shown by Dr. Dawson Turner consists of two metallic rollers with their axes parallel and kept in contact by a spring. One of the rollers has a cam attached to its spindle, and can be made to rotate by means of a small electric motor. Once in each revolution the cam separates the rollers, thus making the break, and at the same time causing the second roller, which rides loose upon its axis, to turn about one-eighth of a revolution. As soon as the cam has passed, the rollers are brought into contact by the spring, and the next break occurs at a different place. The wearing is thus distributed evenly over a large surface. The break is placed in a box containing alcohol or petroleum, and works best when rotating rapidly. An objection to the arrangement is the noise it makes when working. Some experiments were then shown on the discharge of electrified bodies by ultra-violet light. A disadvantage of the electric arc when used to furnish ultra-violet light for use in medicine is that the light is accompanied by heat, so that it is necessary to shield the patient from the heat without interfering with the passage of the light. A condenser spark between iron electrodes is useful because it gives a large amount of ultra-violet radiation without much heat. Dr. Turner showed that this light is capable of discharging bodies whether positively or negatively electrified. He then showed that glass and mica are opaque to the radiation while pure rock salt is transparent.—Mr. Wilson Noble exhibited a mechanical break similar to the one already shown. A roller and a disc, with their axes parallel, are placed in contact and made to rotate in the same direction by a motor. Longitudinal slots are cut upon Ihe surfaces of both, and the break occurs when a slot in the roller comes opposite a slot in the disc. Since the two are moving in opposite directions at their point of contact the break is very sudden. To vary the length of the break without altering the rate of rotation, the slot in the roller is wider at one end than the other, and the disc can be placed so as to touch the roller at any point of its length.—Mr. R. S. Whipple exhibited a temperature indicator for use with platinum thermometers, in which readings are automatically reduced to the gas scale. The instrument is very similar to the well-known Callendar and Griffiths' temperature indicator, with the exception that it is so arranged that the readings obtained are automatically reduced to the gas scale, thus avoiding the necessity of applying a correction. It consists of a simple Wheatstone's bridge with equal ratio arms, the other arms being the thermometer and a long helical bridge wire together with the compensating leads. A travelling contact is moved round the wire until a balance is obtained. The bridge wire is wound on an ebonite drum on the outer surface of which a helix has been cut. The contact piece, which is connected electrically with the galvanometer, is carried from the inside of a cylinder fixed to a shaft. A white celluloid tube on which the scale is divided is fixed to the outer surface of the cylinder. A screw of the same pitch as the helix on the ebonite drum is cut on the shaft, so that by rotating the shaft the contact is caused to travel along the bridge wire, and at the same time the scale is carried past an. index placed above it. The scale has been so constructed that the reading at the index gives directly the temperature of the thermometer reduced to the gas scale. The instrument reads from o° to 1400°C.—Mr. S. A. F. White read a note on the compound pendulum. In the determination of the length of the equivalent simple pendulum for a compound pendulum the form of which is a symmetrical bar and bob with one fixed, one movable knif-edge and no sliding weight it is convenient to make the mass of the movable knife-edge small. In this case, small displacements of this knife-edge wilr not materially alter the position of the centre of gravity or radius of gyration of the pendulum about an axis through its centre of gravity. The time of swing about the fixed knife-edge will therefore remain practically constant. The best determination of the correct position of the movable knife-edge for an equal time of oscillation will be given when for the smallest displacement of this knife-edge there is the greatest variation in the time of oscillation about it. The author has determined the position which makes dt/dh a maximum, h being the distance of the axis of suspension from the centre of gravity. He has also drawn the curve showing the relation between dt/dh and h. The calculations have then been applied to the determination of the position of the movable knife-edge in a particular pendulum. The experimental value of the ratio of h to k deduced from this pendulum when the movable knife-edge is adjusted to its right position agrees well with that predicted by the theory. The author states that when the length of the equivalent simple pendulum is about a metre, it should be possible with a stop-watch reading to 02 second to determine “g” to about .1 or .2 per cent. If the fixed knife-edge were made the movable knife-edge, the value of dt/dh would be very large, but there would be difficulties in the way of measuring the small time of swing and the small equivalent length.
Article PDF
Rights and permissions
About this article
Cite this article
Societies and Academies . Nature 66, 21–24 (1902). https://doi.org/10.1038/066021b0
Issue Date:
DOI: https://doi.org/10.1038/066021b0