2 34 



NA TURE 



[December 23, 1909 



THE PHYSICAL SOCIETY'S EXHIBITION. 

 "TTHE fifth annual exhibition, which was held by the 

 -*■ Physical Society of London at the Imperial College 

 of Science and Technology last week, was marked by some 

 pleasing innovations. Hitherto the exhibition has been 

 limited to the evening only, but on this occasion it was 

 open also in tlie afternoon, and this extension of time 

 was evidently appreciated. Experimental lectures were also 

 introduced. Thus many visitors must have welcomed the 

 opportunity of seeing once again the beautiful and well- 

 known experiments on soap bubbles by Prof. C. V. Boys, 

 F.R.S., as they have not been seen during the last ten or 

 fifteen years. In the evening Prof. S. P. Thompson, 

 F.R.S., showed the remarkable effects obtained by com- 

 bining mica and selenite viewed under polarised hght. 



Turning to the exhibits themselves, we may remark 

 that there were about forty exhibitors, and for this reason 

 it is impossible to do more than give a brief reference to 

 some of the more interesting and novel exhibits in the 

 space at our disposal. In the subject of mechanics, in its 

 broadest sense, Mr. W. V. Gilbert's anticlastic levers, 

 shown by Messrs. Strange and Graham, received the most 

 attention. The device is known by the trade name of 

 " Vilcars," and enables a movement in a given plane to 

 produce a movement in a plane at right angles, although 

 no joints or complicated links are used for producing this 

 change in the direction of motion. 



Among other mechanical devices we may mention the 

 Wimperis acceleromcter, shown by Messrs. Elliott Bros, 

 (see Nature, December 2, p. 139). The acceleration is 

 indicated by a pointer, which moves on a scale suitably 

 graduated. The indication is effected by an eccentrically 

 placed copper disc which is controlled by a hairspring and 

 is magnetically damped. By a special gear and balance 

 weight the disc is compensated so that the instrument is 

 affected only by acceleration in one direction, which is 

 indicated by an arrow on the dial. A simpler type of 

 instrument is that due to Mr. A. P. Trotter, and shown 

 by Messrs. Everett, Edgcumbe and Co. This consists 

 simply of a curved glass "tube nearly filled with liquid, and 

 is best described as a spirit-level of extravagant curvature. 

 When subjected to acceleration along its length the position 

 of the bubble varies according to the acceleration. 

 Obviously itniay be used equally for measuring gradients. 



The principle of the diffusion of gases is brought into 

 practical use in the gas-leakage indicator shown by the 

 Cambridge Scientific Instrument Co. .^n elastic metal 

 chamber is closed by a porous tile, so that the pressure 

 within it increases when the instrument is brought into 

 an atmosphere containing light gas, such as coal gas. The 

 pressure is shown by a pointer, the motion of which is an 

 indication of the presence of gas. 



Ihere was a good display of optical apparatus, including 

 microscopes, cameras, and photometers. Among the latter 

 may be mentioned the davlight illumination photometer 

 due to Mr. A. P. Trotter, and exhibited by Messrs. 

 Everett, Edgcumbe and Co. In testing the illumination 

 of a room by daylight, an absolute figure in, say, candle- 

 feet is not of value by itself, because it depends' upon the 

 brightness of the day on which the measurement is made. 

 This difficulty is overcome by using a vertical tube placed 

 over the screen which receives the illumination to be 

 rtieasured. The _ instrument is first placed where a clear 

 view of the zenith is obtained, and a stop is inserted in 

 thetube so as to cut down the illumination to a con- 

 venient figure. A measurement is then made in the room 

 where desired, the tube having been removed. The stops 

 are so proportioned that the true ratio of the illumination 

 in the room to that in the open is readily obtained. As 

 examples of such measurements the makers mention that 

 the ratio over the Speaker's chair in the House of 

 Commons is o-ooog, and that in the British Museum read- 

 ing room 0007. 



Electrical exhibits were by far the most numerous, and 

 among these perhaps the most interesting was the applica- 

 tion of Abraham's rheograph (shown bv the Cambridge 

 Scientific Instrument Co.) to throw on the screen "a 

 hysteresis loop. It will be remembered that in the rheo- 

 graph a light aluminium frame is suspended in a per- 

 manent magnetic field, and forms the secondary of a small 

 NO. 2095, VOL. 82] 



transformer. The controlling and damping forces are made- 

 relatively unimportant, so that the deflection becomes pro- 

 portional to the current or potential difference under 

 observation. On the present occasion two vibrators were 

 arranged at right angles, one to give a deflection propor- 

 tional to B and the other to H. A beam of light being 

 reflected first from one and then from the other therefore 

 gave a hysteresis loop, which appeared quite steady upon 

 the screen. 



The same firm showed Dr. C. V. Drysdale's slip meter, 

 >i'hich, although it has been in use for some years at the 

 Northampton Institute, has not, so far, been generally 

 available. A circular card, on which certain geometric 

 figures are printed, is fixed to the shaft of the machine 

 under test, and is observed through a stroboscopic disc. 

 The latter is driven frictionally by a conical roller, which 

 in turn is driven by a small synchronous motor. By 

 moving the disc along the roller its speed is varied, and' 

 when the geometric figures on the card appear stationary 

 the speed can be read off on the graduated leading screw 

 which controls the position of the disc. This part of the 

 apparatus is quite small and compact. Depending on the 

 arrangements used, speed, frequency or slip can be deter- 

 mined. 



Messrs. Snell and Tinsley showed one of Dr. C. V. 

 Drysdale's potentiometers for alternating currents, the 

 difficulty of phase difference being eliminated by a " phase 

 shifter," by which the phase of the current in the potentio- 

 meter wire is made to coincide with that of the pressure 

 to be measured. A rotary field is produced in the phase 

 shifter by splitting up a single-phase current into two in 

 quadrature. 



Duddell's vibration galvanometer, which is a modified 

 oscillograph, was shown by Messrs. Nalder Bros, and Co. 

 Messrs. J. J. Griffin and Sons exhibited Mr. Tucker's high- 

 potential primary battery ; it is compact in form, and 

 appears to give its 1000 volts with a minimum of trouble. 

 Mr. F. Harrison Glew showed a radium collector for 

 atmospheric electricity. This consists of a metal spiral 

 coated with insoluble radium salts ; it is suspended in an 

 elevated position from a weather-proof insulator, and as 

 the radium ionises the air around it the wire acquires the 

 potential of the atmosphere, and gives an indication on an 

 electroscope. 



Visitors also had an opportunity of seeing one of the 

 latest methods of transmitting photographs electrically, as 

 exemplified in the apparatus of Mr. T. Thorne Baker, 

 shown by the Daily Mirror. Prof. Horn's method, with 

 selenium cells, has reached a stage of some perfection, but 

 the smallness of the currents available in such a method 

 introduces difficulties. There is thus a tendency to use 

 rather more mechanical methods. In Mr. Baker's 

 apparatus, photographs are printed on fish-glue upon lead 

 foil, and lines are drawn across this so as to expose the 

 Ir.-id. The style comes into contact more or less with the 

 lend, depending upon the density of the image, and thus 

 allows current to flow into the line. At the receiver the 

 picture apnenrs on suitably prepared paper by electrolytic 

 action, and the results seem to be very satisfactory. 



The exhibits we have enumerated by no means exhaust 

 all that was of interest, but want of space renders a com- 

 plete survey impossible. 



EXPERIMENTS ON COMPOUND STRESS. 

 'T'WO papers dealing with experiments on compound 

 stress were read at the Institution of Mechanical 

 Engineers on Friday, December 17. The first of these, by 

 Mr. William Mason, of the University of Liverpool, gave 

 the results of tests made by the author on mild steel tubes, 

 and included tests made in simple compression, in simul- 

 taneous compression and hoop tension, in simple axial 

 tension, simultaneous axial and hoop tension, simple axial 

 compression, and simultaneous axial and hoop compression. 

 Two sizes of tubes were used, viz. 3 inches bore and 

 275 inches bore, of thickness about o.oS and 012S inch 

 respectively. The larger tubes were cold-drawn, and the 

 others were hot-drawn. Some of the tests were made on 

 the tubes unannealed, in others the tubes were annealed. 

 E.Ktensometers of the Ewing type were used in measuring 



