Royal Society Conversazione

The first of the two annual conversaziones given by the Royal Society, that is the one from which ladies are excluded, was held last Wednesday evening in Burlington House, and was attended by some four hundred guests. The exhibition of novel scientific apparatus was not large, and with some notable exceptions, it was not of high interest.

Electrical matters are now always very much to the front at such exhibitions. Professor Crookes repeated some of Mr. Tesla’s experiments, but with simpler apparatus. He used an induction coil, giving twenty complete alternations per second and 467,000 volts, to charge a battery of Leyden jars. The discharges from the battery were sent round the primary wire of an induction coil immersed in oil, while the discharge from the secondary was used to produce the various effects. The vibratory current of the spark of the Leyden jar raised the number of alternations in the final current to one million, it was stated, while the volts were 116,000. The visitors were greatly interested in taking the discharge through their bodies, in confirmation of Tesla’s contention that currents of high frequency are not dangerous. Mr. Crookes showed all the usual phenomena of stream and brush discharges, the lighting of vacuum tubes without conductors, phosphorescence of air, and the like. Professor Oliver Lodge had a number of experiments to demonstrate the behaviour of the electric spark under various conditions. He discharged through air to water, showing how it spread out over the surface seeking for a path, and then he made the discharge take place under water, the result being an exceedingly loud crack and a greatly increased illumination. A shower, with too great a spark length for a strong discharge, excited much admiration, the air being filled with a multitude of violet streams, recalling vividly some photographs of lighting.

Mr. James Wimshurst always prepares some novelty for the Royal Society. This time he had a number of Leyden jars with removable coatings, both inner and outer, made of convolutions of insulated copper wires. The ends of the wire were brought near together, and armed with balls, and it was curious to see the effect of the surging of the charge in the wire. Each time the jar discharged there was an additional discharge in each of the coatings. By combining several jars, very pretty and erratic effects could be obtained.

Of all the apparatus to be seen in the room, nothing exceeded in practical interest the new optical pyrometer invented by Professor H. le Chatelier, and shown by Professor W. C. Roberts-Austen. If the promise of this instrument is fulfilled, and we see no reason why it should not, it will be possible to read the temperature of a furnace or a piece of glowing metal, by merely looking at it through a telescope. The principle of the apparatus is very simple. The light from a standard naphtha lamp is reflected to the eye through a piece of red glass. At the same time the light of the glowing body reaches the eye through another part of the same glass. The aperture through which it comes can be expanded and contracted by means of sliding V-shaped shutters until a uniform illumination of the two parts of the field of vision is attained. Connected with the shutters is an arm moving over a scale. This scale is graduated empirically, either by means of platinum-rhodium thermo-junction pyrometer, or by other means. Of course the new instrument is not available for temperatures below the glowing point.

On the next stand came an exceedingly ingenious and workmanlike chronograph, exhibited by the Rev. Frederick J. Smith. The records are made by means of styli operated by electro-magnets on a vertical smoked paper, which is attached to a carriage running on rails. The time is measured by means of a tuning-fork, which draws a wavy line on the paper. The fork is set in action and the carriage released to make its travel under the influence of a weight. The commencement of the period to be measured is marked by the breaking of an electric circuit, and the drawing of a line on the paper; the end of the period is marked by a second circuit being broken, and another line drawn. The distance between these lines can then be measured by means of a micrometer apparatus, and the time estimated by reference to the known period of the tuning-fork. Mr. Smith had a steel bar a few feet in length, and at intervals during the evening he measured the speed of sound through it. He is also able to measure the modulus of elasticity of metals without mechanical experiments, since the modulus is a function of the velocity of propagation of sound in the metal. The instrument is also furnished with continuous contact breakers to enable photographs to be taken in rapid succession by means of electric sparks, of moving objects, such as falling drops of water, the wings of insects, and the like.

Another chronographic instrument was shown by Captain Holden, R.A. It consisted of a highspeed chronographic pen, with an automatic resetting attachment. The object of this instrument is to take a number of successive records of short intervals of time, on a revolving drum or moving surface, under as nearly as possible the same conditions. The pen is automatically reset after making each record, and the condition of the various circuits in connection with it may be made absolutely identical, and thus the small time error in the working of the apparatus entirely eliminated.

The committee of Kew Observatory have undertaken the examination of photographic lenses, and they showed Major Darwin’s apparatus which has been constructed for the purpose. In bright weather the lens is mounted in a frame, and a picture of a distant object is thrown on a ground-glass screen. The lens is then rotated to a definite angle first to right and then to left, and the focal length is deduced from the measured movements of the object on the screen. At the same time it is noted if the movement of the object is exactly horizontal, and if not its divergence is measured. To try the figure of the lens it is focussed to the edge of a fine ribbon of steel, and the steel is gradually turned towards a flatwise position until it becomes visible. The sooner it is perceived the better the lens. If there is any distortion it is immediately shown in the fine black line in the field of view. The points referred to in the certificate given are principal focal length and of back focus; centering in mount; effective operation of different stops; angle of field of view; curvature of field; definition. When the weather is obscure a collimater is substituted for the distant object.

The new form of eudiometer exhibited by Dr. W. Marcet, is an exceedingly convenient one, especially devised for the rapid and accurate demonstration of the quantity of oxygen contained in air. It consists of a U-tube, the leg in which the mixture of air and hydrogen is exploded being surrounded by a water jacket to maintain the gases at a constant temperature. In using the instrument it is filled with mercury, and a charge of hydrogen is then admitted through a cock at the top of the exploding limb, all the connections being first exhausted of air, and further “washed out” by blowing a current of hydrogen through them into an exhausted india-rubber bag, before the cock on the eudiometer is opened. The connections are next made to the apparatus for supplying the air to be tested under slight pressure, and all traces of hydrogen being driven out of the connecting tubes, &c., a measured quantity of air is admitted to the eudiometer. This air and the hydrogen are then mixed in the usual way by producing oscillations in the mercury column and finally exploded by an electric spark. The details of the apparatus have been well worked out to secure handiness of manipulation and accuracy.

Professor Frank Clowes showed a convenient mode of obtaining in an ordinary miner’s safety lamp a very delicate flame, suitable for testing the presence of minute quantities of firedamp or other inflammable vapour in air. When a lamp is burnt in air containing such admixture there is formed above the flame proper a luminous “cap,” the height of which increases in a certain proportion as the percentage of inflammable gas or vapour in the air increases. When small percentages only of such inflammable gas are present, the “cap” is not readily observable or measurable, unless the flame over which it is formed is of very low luminosity, and such a flame it is the object of Professor Clowes’ device to supply. For this purpose he fits an ordinary miner’s safety lamp with a very small tube which extends from the side of the wick to an external connection on the lamp case, where it can be coupled by a flexible tube to a light steel bottle containing compressed hydrogen. When the atmosphere in which the lamp is burning is to be tested, the hydrogen is turned on and of course ignites from the lamp flame, forming a small jet by the side of the latter. The lamp wick is then turned down until the lamp flame proper is extinguished, leaving the small hydrogen jet alone burning. This hydrogen flame is then adjusted to a standard height, and the luminous “cap” formed over it is measured. The hydrogen flame having a very low luminosity, this “cap” can be observed readily. When the desired measurement has been made the lamp wick is turned up again, when it reignites from the hydrogen flame, and the supply of hydrogen can then be disconnected, and the lamp restored to its normal condition. By the use of this pale hydrogen flame the presence of percentages of firedamp as low as 0.25 or 0.3 can be detected and estimated, while the ordinary flame of the lamp, when carefully adjusted, will only serve to indicate percentages above 3. The application of this mode of testing was shown by a lamp burning in a suitable small testing chamber containing air with which a small percentage of gas had been mixed, the “cap” on the flame being seen through a window in the side of the testing chamber.

Messrs. Pyke and Harris showed an alternate current dynamo in which there were no sliding electric contacts. Both the field and the armature coils are stationary. The variations of the magnetic circuit are obtained by means of revolving magnetic inductors which move between two sets of stationary poles wound with coils. The field is produced by a single coil surrounded by magnetic material both on its inner and outer circumference and on one face. Its poles are formed as laminated rings with projecting pole pieces, the two rings being concentric with the poles projecting towards each other. On these poles are placed coils, and between them the inductors pass. An even number of polar projections are employed, and the revolving inductors are arranged so, that in proportion as one pole is being demagnetised, the magnetism of the other poles increases, and vice versa, thus the total magnetic effect being always approximately constant. The induced current conductor is so arranged that the polar projections which are being magnetised, and those which are being demagnetised, have a similar inducing action upon it, the magnetising of one set of polar projections thus increasing the effect produced on the same wire by the demagnetisation of the other set of polar projections. The machine can be constructed for single or many phase alternating currents.

Mr. Cecil Carus-Wilson showed samples of musical sands from New Zealand, and also of some artificial musical sands which he had succeeded in producing. Some of the latter had been “killed” by constant exhibition, and gave only a dull sound in place of the musical note.

There were two very popular exhibits downstairs. Professor C. V. Boys showed on the screen photographs of flying bullets. These are obtained by throwing the shadow of the bullets, obtained by an electric spark, on to a sensitised plate. The bullet in its course completes a circuit between a lead and a copper wire and effects the discharge of a small Leyden jar. This brings about the discharge of a larger jar, and gives the spark which produces the photograph. The pictures were most interesting, indeed they surpassed anything of the kind ever yet shown. In front of the bullet there was a wave of compressed air, and behind it another, each clearly indicated in the photographs. The bullet also left a wake behind it, caused by the efforts of the air to fill the space in its rear. The pictures of bullets passing through sheets of glass were much appreciated. At the rear of the pane the particles were flying off as if by the result of an explosion, while in the front the disturbance was much less. The air waves caused by the vibration of the glass could be seen running down in front of it, and changing from compression to rarefaction as the nodes occurred in the glass. One picture represented an aluminium bullet fired by smokeless powder from a magazine rifle with a speed of 3000 ft. per second. It had pierced a sheet of paper, and was followed by the scraps which it had cut out. Each of these was surrounded by its own compression waves of air, and it was noticeable that where these met they did not cross like sound waves, but united and flowed away at their mean angle. One of the bullets had a number of holes pierced through it at right angles to its axis, and through one of these the light could be seen, demonstrating that the photograph had been taken in one two-hundred-thousandth of a second. The marvellous powers of the photographic plate were signally shown by this exhibit, which was intensely interesting.

At eleven o’clock there was a demonstration of composite heliochromy by Mr. F. E. Ives, of Philadelphia. Three photographs are simultaneously taken by one camera on one plate. By means of light filters one of the negatives is made by such light rays only as excite the fundamental red sensation, and in due proportion; another by light rays as they excite the fundamental green sensation, and another by light rays as they excite the fundamental blue-violet sensation. These three pictures are all reproduced through coloured glasses and superposed on the screen, giving a coloured representation of the original object which is wonderfully natural. The same result can also be obtained by means of a device the size of stereoscope, being three optical systems with red, green, and blue glasses. The three images being exactly superposed appear as one only, in which the natural colours are reproduced, together with the light and shade. The effect is exceedingly good, and the process seems full of promise.

Among the minor exhibits was a lecture-room apparatus by Professor T. E. Thorpe to show the phenomena of dust explosion; vacuum tubes without electrodes by Dr. J. T. Bottomley; an apparatus for testing for incompleteness of colour vision by Mr. R. Brudenell Carter; and photographs of stellar spectra by Professor J. Norman Lockyer. There were also some very interesting archæological remains from Mashonaland, but they do not fall within the scope of this article.