226 



NATURE 



[August: 1 8, 1910 



are made to revolve in 4J seconds instead of a nominal 5. 

 A check comes into play at the end of the revolution, and 

 the cylinder is stopped until the 5 seconds are completed, 

 the motor working against a friction clutch in the ordinary 

 way during the stop. At the end of the fifth second each 

 cylinder is automatically released by chronometric means, 

 in the manner shown in the next diagram (Fig. 12). 



Here you will see that a special form of clock is used, 

 with a centre seconds' hand which projects beyond the 

 face by about an inch, and to the end of it is attached 

 a brush of exceedingly fine silver wires. At every twelfth 

 part of the circumference of the clock dial is fixed a 

 platinum pin, and consequently every five seconds the 

 little brush wipes against the convex surface of one of 

 them. Each of these pins is connected with one terminal 

 of a battery B, the other side of the battery leading to the 

 relay R, as does also the centre seconds' hand. There- 

 fore each time the brush wipes against a pin the circuit 

 is closed, and the relay throws into action the local circuit 

 connected up with the terminals TT. This circuit excites 

 an electromagnet, which attracts an armature and pulls 

 away the check which is holding back the cylinder. At 

 the end of each 5 seconds the cylinders consequently re- 

 commence turning. 



Well calibrated clocks of the pattern used will keep 

 good time for the period taken to transmit a picture, one 

 gaining on the other quite an inappreciable amount, 

 depending on the friction of the brush against the pins. 

 By this means the two cylinders are kept in very fair 

 synchronism independently of any wireless communication, 

 and the less the interval between the stopping and restart- 



ing of the cylinders be made, the more accurate and satis- 

 factory will be the effect. 



The other method of synchronising is controlled by 

 electromagnetic oscillations. Let us suppose that a 

 coherer is being used as cymoscope ; the transmitting 

 cylinder is kept running without any interruption, but by 

 means of a fleeting contact it sends out a wave at the 

 conclusion of its turn, a bare space in the picture being 

 necessary about half a second beforehand, so that no waves 

 are sent out for the half-second previously. The receiving 

 cylinder is driven too quickly, and checked at the end of 

 the revolution. It then, by means of a cam pressing 

 down a spring lever, throws out of circuit the marking 

 current, and brings into circuit the relay which actuates 

 the electromagnetic release. Consequently, when the 

 synchronising wave is received, the coherer causes the 

 relay to work, the release is effected, and the receiving 

 cylinder starts a new revolution in unison with the 

 transmitter. 



This means of synchronising is only possible in cases 

 where a cymoscope is employed that is capable of actuating 

 a relay, and you will therefore see that it is out of the 

 question, except for short distances. I am therefore using 

 the chronometric system already described in the apparatus, 

 and it is being embodied in the quartz fibre apparatus I 

 am now about to describe. I must first remark that the 

 wireless work has been greatly facilitated by the courteous 

 assistance so readily given by the Marconi Company. 



The general form of the Einthoven galvanometer is well 

 known, and the modified type of it used by Prof. Korn for 

 photo-telegraphic purposes has been already shown. If, 

 now, we make the magnetic field very much more intense 



NO. 2129, VOL. 84] 



by building the field magnets heavier, and using a large 

 number of ampere turns in the winding, and also employ 

 a " string," which is very much more elastic than the 

 silver ribbon, the displacement of the string will be corre- 

 spondingly greater. The silvered quartz fibre used by 

 Duddell for this purpose gives an extremely sensitive 

 instrument, and very appreciable displacement is obtained 

 with the current from one dry cell passing through 35 to 

 90 megohms resistance. 



It is not long since Prof. Fleming explained at this 

 institution the valve receiver for detecting wireless oscilla- 

 tions ; in ordinary wireless telegraphy, the minute 

 alternating currents are rectified, and sounds are heard 

 in the telephone in circuit owing to small unidirectional 

 currents. If these currents be passed through the silvered 

 quartz string of the galvanometer, the string is shifted. 

 If, therefore, we cause a shadow of the string to lie over 

 a fine slit, any displacement will cause the slit to be 

 opened, as it were ; the shadow will be shifted off the slit, 

 and light will be free to pass through it. Oscillations 

 corresponding to the lines in a photograph or sketch could 

 therefore Be utilised to cause shifting of the shutter in the 

 manner I have already described for Korn's telautograph, 

 and a sensitive photographic film could be revolved on a 

 drum beind the slit to receive the picture. Such an 

 apparatus is now in course of preparation ; but the amount 

 of light that passes through the slit is extremely small, 

 owing to the fineness of the fibre. Mr. Sanger-Shepherd 

 has therefore attached a minute shutter to the fibre, cross- 

 ing the optic axis ; this enables me to use a very much 

 wider slit, and also to adopt the alternative procedure for 

 reception, which you will now see represented in the 

 diagram on the screen. 



For photographic reception, the oscillation is passed into 

 the valve detector, and thence to the quartz fibre AB, 

 which is stretched across the field of the magnets (not 

 shown), the poles of which are bored with a tunnel, tTirough 

 which the beam of light is directed. When the fibre is 

 displaced, light is enabled to pass through a fine slit W, 

 and so act on the photographic film. Where, however, 

 the shutter is attached to the fibre, a much wider slit 

 can be used, and then a pair of narrow compensated 

 selenium cells SS are placed behind the slit W, a positive 

 lens being interposed. When a signal corresponding to 

 a dot in the photograph {i.e. the traversal of a line by 

 the stylus) is received, the fibre shifts, light falls on the 

 cells SS, and their resistance is decreased sufficiently to 

 enable the battery E to actuate the relay R. This closes 

 a local circuit, "in which the telectrograph receiver is 

 included, and a mark appears on the paper. In this way 

 a visible record is obtained, which greatly facilitates the 

 process. 



Wireless photo-telegraphy may eventually prove of more 

 utility than the closed-circuit methods, because it would 

 bring America within reach of this country, and would 

 enable communication to be made where telephone or 

 telegraph lines did not exist. It is not limited to photo- 

 graphs — banking signatures, sketches, maps, plans, and 

 writing could be transmitted. But I would point out most 

 particularly that the work is as yet in the very earliest 

 stages, and that in giving you some account of it to-night 

 I may be bringing before your notice methods and systems 

 on which a few years hence you will look back with a 

 smile — as curious merely from a historical point of view. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



The inaugural address on the occasion of the opening 

 of the winter session of the London School of Tropical 

 Medicine will be delivered by Prof. H. A. Miers, F.R.S., 

 principal of the University of London, on Friday, 

 October 14. 



The calendar of the faculty of medical sciences of 

 University College, London, is now available. The 

 college is a university centre for preliminary and inter- 

 mediate medical studies, and its faculty of medical sciences 

 comprises the departments of physics, chemistry, botany, 

 and zoology (the preliminary medical sciences) ; also the 

 departments of anatomy, physiolog)', and pharmacology 

 (the intermediate medical sciences), and the departments of 



