PHOTOTELEGEAPHY ^AKMAGNAT. 



201 



are wound, parallel and insulated from each other, two fine platinum 

 wires, 1 and 2. Upon one face this slab is covered with a very thin 

 coat of selenium so that these two platinum wires are now connected 

 through the selenium and this separating resistance can be doubled or 

 even more than doubled by removing the cell from the light to dark- 

 ness. It is also extremely imj^ortant to protect these cells from the 

 ordinary variations of temperature during an experiment as their 

 resistance varies from both light and heat. 



The string-galvanometer used by Korn is a very recent device and 

 was first made a few years ago by Ader as a receiver for submarine 

 telegraphy. It possesses a very great sensibility and Einthoven has 

 since constructed one which will detect currents of the order of 

 10-^- amperes — a millionth of a microampere. 

 Reduced to its simplest, schematic form, the 

 string-galvanometer consists of a thin conduct- 

 ing thread, / (fig. 4), stretched between two 

 fixed points, a and Z>, and passing between the 

 poles, PP, of an electro or a permanent magnet. 

 A current passing through this thread causes it 

 to bend in a direction perpendicular to the 

 lines of force of the field and this deflection is 

 observed with a microscope whose axis coin- 

 cides with the direction of the field. Einthoven 

 used a thread of silvered quartz having a rather 

 large electrical resistance and a long period of 

 oscillation. Korn constructed it of bronze, and 

 as it must carry the diaphragm used for vary- 

 ing the light, it had rather large dimensions. 



In the first trials Korn, as did all those 

 Avho had preceded him, used the total variation 

 of the resistance of the selenium, but he early 

 saw it was impossible to obtain in that way rapid signals on account 

 of a certain inertia which the selenium had in following the varia- 

 tions of resistance impressed upon it by the light. The resistance of 

 the selenium at each instant depends not only on the illumination to 

 which it is then exposed, but as well upon its previous illumination, 

 and if we wish the cell to return to its normal resistance in the dark 

 it must be given a considerable time. Korn overcame this difficulty 

 through a method of compensation which used the small differential 

 variations in the resistances of two cells — one, e, which is a part of 

 the transmitting system and is submitted to the direct action of the 

 light (fig. 2), the other which serves in the receiver and is placed 

 before the auxiliary lamp, o, the intensity of the light of which is 

 varied by the string-galvanometer, q. 



