TELEGRAPHIC APPARA TUS. 
2i 7 
cases a water resistance is interposed in the receiving circuit. 
This is formed of two tubes (fig. 155) within which hangs a 
gutta-percha covered wire terminated by pieces of platinum. 
According as the wire is raised or lowered, the resistance of the 
circuit is increased or diminished, and thus the range of the 
movement of the spot of light can be limited as desired. Some 
telegraph clerks like the signals very small, while others prefer 
to have them more ample ; this apparatus enables each to 
gratify his particular taste. 
On long lines it is necessary to interpose a condenser in 
the circuit, so as to do away with the effects of induction. 
This condenser is placed either in the circuit of the apparatus 
itself, oris placed at the beginning of the line itself, which then 
