TELEGRAPHY OF PHOTOGRAPHS BAKEE, 



263 



prism inside the cjdinder on to the selenium cell, throngh 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. These 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 it shifts aside, allowing light from a Nernst lamp No to enter 

 the prism P, whence it is reflected on to the second cell SS. The tele- 

 phone lines connecting the two instruments go direct to the wires of 

 a similar galvanometer, which is in series with the galvanometer of 

 the transmitting instrument. If w^e 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 be- 

 comes of great use for many purposes. Prof. Korn's method of com- 



FiG. 2. 



pensation is to let the light fall at the same time on two cells of op- 

 posite characteristics; one has great inertia and small sensitiveness, 

 the other low inertia and great sensitiveness. By using the two cells 

 on opposite sides of a AVlieatstone 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 very rapidly instead of graduall}^, wdiilst 

 when it is suddenly shut off (at P in the upper curve) it drops to 

 zero almost instantly instead of falling gradually. 



I shall now show, by means of a meter, an image of the pointer of 

 W'hich will be projected on to the screen, how the inertia of selenium 



