NATURE 



[August i8, 1910 



as many as three hundred sharply defined chemical marks 

 can be recorded in one second by its means. The method 

 of application will be seen if we have the last slide shown 

 again (Fig. 4) ; here, shunted on to the line (which is closed 

 by the stylus Sj and the metal drum), is a circuit con- 

 taining two batteries, B, and Bj, and the two sections of 

 a divided looo-ohms resistance, W, and W,. Shunted 

 across the variable contacts of the resistances is a variable 

 condenser K. By varying the resistances, Wj and Wj, we 

 can vary the power of the current used to sweep out the 

 residuary charges in the line ; the current can, of course, 

 fiow through the chemical paper on the drum, but the 

 pole of the battery Bj, connected to the style, is of opposite 

 sign to that of the line unit connected to it. 



"when the leakance on the line is great and evenly dis- 

 tributed, less reverse current from the balancer is neces- 

 sary, this being quite in accordance with Heaviside's 

 formulae for telephony over lines with capacity and in- 



ductance. It is interesting to note, also, that by increas- 

 ing the voltage of the reverse batteries B, and B,, con- 

 siderable greater contrast can be obtained in the pictures ; 

 the finer the half-tone screen employed in splitting up the 

 photograpFis into lines, the higher, again, must the voltage 

 of B, and B, be made. 



I should like to take up a few moments in referring to 

 the actual utility of photo-telegraphy. The demand by the 

 public for illustrations in their daily papers must be 

 admitted. News is telegraphed in order to expedite its 

 publication, and photographs illustrating this news can 

 therefore be telegraphed advantageously. But where a 

 large installation and establishment, with accumulators, 

 a large instrument and an operator to 

 work it are required, the cost of telegraph- 

 ing ever\" individual picture becomes quite 

 out of proportion to its value. It is there- 

 fore desirable to direct especial attention 

 to the portable instruments, the first one 

 of which is shown for the first time to- 

 night. \ photographer going to obtain 

 pictures of some important function or 

 interesting event can take the machine 

 with him, prepare his pictures, and tele- 

 graph them to his head office, and when 

 the event is over he simply returns with 

 the apparatus. For criminal investigations the port- 

 able instrument will, I feel sure, become of consider- 

 able value also. Through the continued courtesy 

 shown by the Postmaster-General and Major O'Meara, 

 the engineer-in-chief, we have been given every 

 facility for developing the work, and I believe that the 

 uses of the portable instrument will before long have been 

 amply demonstrated. 



If a picture revolving beneath a tracer has to re-draw 

 itself, as it were, on a piece of paper perhaps hundreds of 

 miles away, it is obvious that each mark re-drawn must 

 occupy a precisely similar spot on the new paper as it 

 does in the original picture, .^s cylinders or drums are 

 used in picture telegraphy, this means that they must 

 revolve in perfect unison. If one drum were to gain on 

 the other we should have, in the case of a portrait, a nose 



being recorded where the eye ought to be, or something 

 equally disastrous ; in fact, if the two machines get the 

 least bit out of step, the received picture is completely 

 ruined. The method of synchronising used by Prof. Korn 

 has proved very satisfactory, and has been adopted in 

 practically all systems of photo-telegraphy. The motors 

 which drive each drum are run at about 3000 revolutions 

 per minute, and geared down very considerably, so that 

 the drums themselves revolve, perhaps, at 30 revolutions 

 ner minute ; the motors are run from secondary batteries 

 of ample capacity to ensure smooth working, and should 

 be run for a sufficient time before beginning a transmission, 

 to allow of their warming up. 



The speed of each motor is controlled by a regulat'ng 

 resistance in series with the field magnets, and the speed 

 is ascertained by means of a frequency meter, which 

 indicates the number of revolutions per second. The dial 

 of this meter is shown on the screen. -A set of tuned 

 steel tongues are fixed in front of a mag- 

 g net, which is supplied with alternating 



current obtained from slip rings on the 

 motor, and each tongue has a different 

 period of vibration. When the alterna- 

 tions in magnetism correspond with the 

 period of vibration of any one spring, that 

 spring vibrates, and thus serves as an 

 indication of the speed of the motor. 



The receiving drum is revolved a little 

 _ quicker than the transmitting drum. It 



^ consequently completes its revolution before 



the transmitter. It is then stopped by a 

 steel check, and is obliged to wait until 

 the other drum has caught it up. When 

 the transmitting drum has completed its 

 turn, a fleeting contact comes into play, 

 a reverse current is sent to the receiving 

 instrument ; this is led into a polarised 

 relay, which actuates an electromagnet, 

 and this magnet removes the check. 

 Thus, however much one drum gets out of step with 

 the other, the fault is limited to each revolution, and both 

 drums must always start off in unison for each new revolu- 

 tion. I have found that where each operator endeavours 

 to keep his motor running uniformly by regulating the 

 resistance according to the fluctuations recorded by the 

 frequency meter, the personal element makes itself visible 

 in the results ; straight lines appear wavy, and the 

 synchronism is not at all good. I therefore tried very 

 carefully calibrating the motors by timing first, and then 

 arranged that, once started, the motors should not be 

 touched ; the gain in speed of each is approximately the 

 same if both motors are run from secondary batteries of 



the same ampere-hour capacity, and in this way we have 

 obtained the most perfect results as regards synchronisa- 

 tion. 



The great advantage of this process is that the whole 

 operation is in full view, whereas with systems in which 

 the received picture is obtained on a photographic film 

 one has to develop such film before it is possible to dis- 

 cover whether anything is wrong. With the receiver 

 described, the operator keeps his hand on the sliding con- 

 tact of the resistances, and merely adjusts their position 

 during the first two or three seconds, according to the 

 condition of the electrolytic marks, i.e. whether crisp and 

 concise or not. The transmitting cylinder can be used as 

 the receiving cylinder, and the apparatus is thus reduced 

 to the limits of simplicitv. 



Towards the end of last year I designed a portable 



NO. 2129, VOL. 84] 



