82 PHOTO-TELEGRAPHY 



of accomplishment as the former. The silver string 

 in the galvanometer can be made finer ; I have 

 obtained them in this country less than 2^00^ i ncri i n 

 diameter., while by using the silvered quartz threads 

 suggested by Duddell for the Einthoven galvano- 

 meter a thread j^Joo^h i ncn diameter can be 

 employed. The magnetic field can also be greatly 

 increased by building the magnets considerably 

 larger and making the windings take enough 

 current for complete saturation of the iron. The 

 instrument then becomes exceedingly sensitive, 

 and it merely remains for the optical parts to be 

 suitably constructed. 



As explained elsewhere, the string of the galva- 

 nometer should have a period which is neither equal 

 to nor a multiple nor sub -multiple of the period of 

 the interruptions due to the transmitting apparatus ; 

 this applies only to the transmission of half- 

 tone line photographs, which consist of a definite 

 number of lines per unit of length. The natural 

 period of the string can be varied in several ways. 

 It can most readily be measured by means of the 

 recording apparatus described in Chapter V. If 

 a short current be sent through the string by means 

 of suddenly tapping a Morse key, a jerk is given to 

 it and it is displaced through a distance d (at the 

 centre). It then swings back to zero and then past 

 the zero point, to a point distant d x from it 

 where d x is less than d. One has, in fact, a 



