June 30, 1898J 



NA TURE 



211 



but it is consoling to think that the experience gained that 

 night will undoubtedly be used to render the work at the time 

 of the next November meteor shower more efficient. As regards 

 the best form of lens to employ, Prof, Pickering advocates " a 

 portrait lens of the kind formerly used by photographers." The 

 plates should be exposed for half an hour, and at the end of 

 this time the camera should be again pointed to the region 

 fifteen minutes preceding the radiant point. At the Solar 

 Physics Observatory, at Kensington, the methods employed 

 were to fix a camera on the tube of a large equatorial (near the 

 object-glass end), and also on the hour-angle circle ofasiderostat, 

 both cameras being moved by the clock-work of each instrument. 

 Other cameras were fixed and oriented towards the radiant 

 point and other directions. The 9-inch equatorial with objective 

 prisin was also used, but no bright meteor, unfortunately, passed 

 across the field. May the meteorological conditions be such 

 that the meteor shower of November 1898, will be well observed 

 and successfully photographed ! 



HIGH SPEED TELEGRAPH TRANSMISSION 

 BY MEANS OF ALTERNATORS. 



A LTHOUGH, at the present day, high speed transmission is 

 -'*• much more limited in its application than at an earlier 

 period in the history of telegraphy, owing to the commercial 

 aspects of the question having been unavoidably altered, 

 attempts have been made from time to time to produce im- 

 provements in this direction ; but until lately the admirable 

 .system invented by the late Sir Charles Wheatstone, and con- 

 siderably improved by the British Post Office Telegraph Ad- 

 ministration, has been the best available method of automatic 

 high speed signalling. 



The speed at which a series of waves can be passed over a 

 given line depends primarily and inversely upon the product of 

 the total resistance into the total capacity, the form of the wave 

 having a considerable influence on the speed where any measur- 

 able capacity is present. 



In the ordinary Wheatstone automatic fast speed system of 

 telegraphy, the letters are formed by waves of different duration, 

 a dot being produced by a short wave, a dash by a longer one. 

 This renders it necessary to charge the line longer for a dash 

 than for a dot, which is a grave defect in fast speed working; but 

 the condenser compensation, introduced and employed by the 

 British Post Office, practically doubles the speed attainable on 

 any given line by, in some measure, equalising the line charges. 

 That is to yay, the condenser used is always of a capacity which 

 admits of a full charge during the time interval of a dot, and 

 a current of the duration of a dash does not give the condenser 

 any higher charge. Indeed condenser compensation has such 

 a beneficial effect, that the defect of unequal impulses is 

 almost overcome, inasmuch as the increase of speed obtained 

 by this arrangement and equal impulses, is only 5 per cent, 

 greater than that obtained with currents of unequal duration. 

 Again, although the signals be made equal in this system, 

 another difficulty presents itself; that is, the waves that are sent 

 through the line are the results of the sudden applications of the 

 full E.M.F. used (in practice loo volts), and consequently a 

 reversal means a sudden change of 200 volts, i.e. from 100 volts 

 positive to 100 volts negative. The form of the current wave with 

 such a system depends almost entirely on the nature and form of 

 the circuit. It is easy to produce corre.spondingly sudden and 

 complete changes in the current when the circuit possesses 

 only resistance, but when capacity, &c., is present, the form 

 of current wave is vastly different to the impressed E.M.F. 

 wave; for example, take the letter "A," the actual current 

 curve on a land line without condenser compensation is shown 

 in Fig. I, while Fig. 2 represents the effect of shunted condenser 

 compensation. 



Prof. A. C. Crehore, of Dartmouth College, U.S.A. in con- 

 junction with Lieut. G. O. Squier, of the United States 

 Artillery, have, however, been led to make some experiments 

 with alternators, and have suggested a mode of high speed 

 signalling which, although presenting some mechanical diffi- 

 culties, has recently been tried by the inventors of the Post 

 Office telegraph lines in England, under the direction of Mr. 

 I'reece, and found to produce a distinct increase of speed. 



Fig. 3 .shows an ordinary sine wave as produced by an alter- 

 nator, and it is this form of wave that Messrs. Squier and 



Crehore use in their so-called "synchronograph" system of fast 

 speed telegraphy. 



The signals are obtained by the omission of certain complete 

 cycles or semi-cycles, the message being read by means of the 

 blanks in the regular succession of recorded dots ; or signals can 

 be recorded on chemically prepared paper. 



This system is to some extent a synchronous one with this 

 great advantage oyer the many well-known synchronous systems, 

 that the synchronism is not required between the transmitter at 

 one end and the receiver at the other end of a line, but between 

 the alternator and transmitter at the sending end of the line. 

 This is easily obtained by driving the transmitter from the 

 generator shaft. The transmitter itself is exceedingly simple, 

 and consists of a wheel the circumference of which is one con- 

 tinuous conductor, presenting a smooth surface for the brushes 

 to bear upon. If the periphery of this wheel be divided into 

 forty equal parts, and be geared to run at one-fourth the speed of 

 the armature of a ten-pole alternator, clearly one of these equal 

 parts will correspond to one semi-cycle of E.M.F. produced by 

 the alternator. Upon the surface of the wheel bear two 



Ztpo li 



NO. 1496, VOL. 58] 



Zero line 



----- ^v^^^^-- 



Fig. 



brushes, carried by an adjustable brush-holder. One brush 

 is joined to the generator, and the other to the line, so that 

 the current entering one brush from the generator passes 

 across the transmitting wheel to the other brush, and thence out 

 to the line. 



Now if a piece of paper -^^ of the circumference of the wheel 

 be fixed thereon in such a position as to pass under one of the 

 brushes, one semi-cycle or half-wave of current will be omitted 

 in every twenty complete waves, and by means of a suitably pre- 

 pared paper ribbon, or " slip," any combination of signals can in 

 this simple manner be transmitted. The brushes are adjusted so 

 that the periods of disconnection and connection coincide with 

 the zero points of E.M.F. The transmitter may, however, have 

 only one brush joined to line, and the wheel itself may be made 

 the connection to the generator. With this mode of signalling 

 much higher E. M.F.s maybe used, and connections and dis- 

 connections made almost without spark at the brush contacts. 



The speed of the transmitting wheel with respect to the 

 generator shaft is immaterial, the essential being that its cir- 

 cumference should contain an integer number of times the arc 

 which a point fixed with respect to the field would describe on 

 such circumference during one semi-period of current. 



Zero line 



Fiu. 3. 



Complete control of every semi-cycle of current thus permits 

 the maximum speed of transmission of signals with a given 

 frequency. If the transmitter does not act in synchronism with 

 the generator, the "make" and "break" of the circuit occurs 

 when the current is not naturally zero, and considerable inter- 

 ference results ; care is, therefore, taken to ensure that the "slip" 

 admits of the line connections being made at the proper times 

 only.- 



Athough the received signals were originally intended by 

 Messrs. Squier and Crehore to be recorded on chemically pre- 

 pared paper, they have also devised a very ingenious massless 

 receiver, although at present it is not in a practical form. It is 

 based on the well- known discovery of Faraday that a beam of 

 polarised light may be rotated by means of a magnetic field, the 

 direction of rotation of the ray being the same as the direction 

 of the current producing the field ; the rotary power depends 

 upon the intensity of the magnetic field, and the total amount 

 of rotation upon the length of the rotary medium in which this 

 magnetic field exists and through which the ray passes. 



The method adopted is to pass a beam of light through a 



