336 



SCIENCE PROGRESS 



curve varies with each individual cell, and the shape of the 

 aperture, i.e. the obtuseness of the angle, must be altered to 

 suit the cell. 



The time of the transmission is long by this method, and for 

 commercial value it can hardly be compared with the two 

 methods now to be described. 



Mechanical Methods of Transmission. 



The idea of a telegraphic transmitter for photographs con- 

 sisting of line drawings or " half-tone " prints on metal attached 

 to a cylinder revolving beneath a metal tracer or style is a 

 very old one. Yet until Prof. Korn adapted to such a trans- 

 mitter the " string " galvanometer nothing of a really practical 

 nature was ever accomplished. The arrangement of his 

 telautograph can be seen at a glance from fig. 5. Here c is 

 the metal cylinder of the transmitter, which revolves spirally 



■as 4--© 



Fig. 5. 



in the manner ol a phonograph cylinder under the steel 

 tracer r. On the cylinder is put a piece oi copper foil on 

 which a sketch has been drawn with some insulating ink, such 

 as shellac solution tinted with an aniline dye. b is a battery 

 of from thirty to sixty volts ; one element of this is connected 

 to the style, the other to the line, the cylinder (or base of 

 the instrument) being earthed, or attached to the second unit 

 of a telephone line. The received current, which is of course 

 interrupted every time a shellac line comes between tracer and 

 cylinder, passes through a flat silver wire w stretched across the 

 field of an electromagnet with bored poles, as in the case of 



