August i8, 1910] 



NATURE 



machine, two of which Mr. Sanger-Shepherd has just 

 completed, embodying in them a number of improvements 

 of his own, and these machines, which have worked 

 successfully on their trials, are shown on the lecture table 

 to-night. They are suitable for line or wireless work, and 

 will, I believe, prove of great value in naval and military 

 oi'.erations. 



The Daily Mirror inaugurated the Paris-London photo- 

 graphic service in November, 1007, with Prof. Korn's 

 selenium instruments, which I shall briefly describe, as 

 Korn is now making two new selenium apparatus with 

 the view of transmitting photographs from New York to 

 London. In this system use is made of tlie fact that the 

 electrical resistance of the metal selenium varies accord- 

 ing to the strength of illumination to which it is subjected, 

 a beam of light passed through the light and dark parts 

 of a photograph in succession being used to vary the 

 strength of an electric current sent to the receiving 

 apparatus. 



In Korn's selenium transmitter light is concentrated from 

 a Nernst lamp to pass through a revolving glass cylindei, 

 round which a transparent photograph (printed on cellu- 

 loid) is fixed, the beam traversing the film at its brightest 

 part, where the rays come to a focus (Fig. 6). The light 

 which passes through the picture is reflected by a prism 

 inside the cylinder on to the selenium cell, through which 

 the current passes. Across the circuit is shunted a 

 galvanometer of the Einthoven pattern, containing two fine 

 silver strings free to move laterally in a strong magnetic 

 field. 'Ijiese are represented by AB, the magnet poles 

 being MM. When a bright part of the photograph admits 

 of light falling on the sensitive cell, current passes through 

 AB, and i: shifts aside, allowing light from a Nernst lariip 

 N, to enter the prism P, whence it is reflected on to Ihe 

 Mcond cell 3?. The telephone lines connecting the two 

 Instruments go dir.-ct to the wires of a similar galvano- 

 meter, which is in series with the galvanometer of the 

 transmitting instrument. If we imagine MM to be the 

 receiving galvanometer, then we remove the prism P, and 

 the light acts on a sensitive photographic film attached 

 to the drum C, which revolves synchronously with the glass 

 cylinder of the sending instrument. 



The inertia of selenium, once overcome, the metal 

 immediately becomes of great use for many purposes. 

 Prof. Korn's method of compensation is to let the light 

 fall at the same time on two cells of opposite character- 

 istics ; one has great inertia and small sensitiveness, the 

 other low inertia and great sensitiveness. Bv using the 

 two cells on opposite sides of a Wheatstone bridge, dividing 

 the battery into two parts for the other sides, the deflection 

 in the galvanometer is very rapid. You will see the effect 

 from the two curves now shown on the screen ; that above 

 the axis along which exposure is measured is the sensitive 

 cell, that below this axis the cell of low sensitiveness. 

 Clearly the current passed through the galvanometer is 

 that obtained by joining the sums of the ordinates. This 

 gives the small curve shown as the shaded portion. 'When 

 the illumination is thrown on the cell the current rises 

 verv rapidly instead of gradually, whilst when it is 

 suddenly shut off (at P in the upper curvel it drops to 

 zero almost instantly instead of falling graduallv. 

 NO. 2129, VOL. 84] 



I sliall now show, by means of a meter, an image of 

 ihe pointer of which will be projected on to the screen, 

 how the inertia of selenium is overcome. You will first 

 see that if I take away the screen so as to allow light to 

 fall on the selenium cell, current passes into the galvano- 

 meter, and the needle slowly deflects several degrees. 

 Now, I quickly shut off the light by intercepting it with 

 the screen, and the needle comes 

 slowly backwards. Such sluggish 

 movement would be impossible for 

 the purposes of photo-telegraphy, 

 where at least half a dozen changes 

 per second are required to be re- 

 corded abruptly even in transmitting 

 the simple portraits to which the 

 selenium process is limited. 



Now, using two cells of different 

 characteristics and a Wheatstone 

 bridge arrangement, I will once 

 more allow light to fall suddenly on 

 the two cells simultaneously, and 

 you will see that the galvanometer 

 needle records the change in resist- 

 ance of the combination quite 

 quickly ; the combination is even 

 more noticeable when the light is 

 suddenly shut off again, the needle 

 returning to zero with great 

 rapidity. This -compensated arrangement of selenium 

 cells at once renders their use of practical value for various 

 physical and optical measurements. Prof. Korn has found 

 that for an increase in the illumination 51, the current 



obtained is given by the equation y = a.5I.e-P' '", where 

 V is the current, o the sensitiveness of the cell, ^ and m 

 its inertia constants, and e the base of Naperian 

 logarithms. For two cells to be combined to the greatest 

 advantage, we must have them such that if their equations 

 are respectively 



ST ,-^1'"'^ 



;/, = 0i5Lf 



and 



1_ 



y.,^a.M.e-l^-"'' ' 

 then 



at 

 This makes the condition for good compensation that 



m is usually almost constant, and w^ith suitable Giltay cells 

 is about two-thirds. 



CuRvc 5»<ow/i/5 ericr Of ricBi i 



Pf f,/<.rA/\/ri ./ Z[ Li NIUM 



Fig. 7. 



In practical language, the condition for compensation is 

 that the principal cell should have great sensitiveness and 

 a small inertia constant, the compensation cell low 

 sensitiveness and a high inertia constant, the product of 

 sensitiveness and inertia constant being the same in the 

 case of both cells. 



The physical properties of selenium are of such import- 



