Wave- length of Electric Radiation by Diffraction Grating. 173 



various Substances for the Electric Ray," ' Roy. Soc. Proc.,' vol. 59, 

 p. 163). The receiver is made linear by arranging bits of steel spiral 

 springs side by side, the sensitive surface being 3 mm. broad and 

 2 cm. in length. An electrical current enters along the breadth of 

 the top spiral and leaves by the lowest spiral, having to traverse the 

 intermediate spirals along the numerous points of contact. The 

 resistance of the receiving circuit is thus almost entirely concentrated 



FIG. 5. 



The Spiral Spring Coherer. 



at the sensitive contact surface, there being little useless short cir- 

 cuiting by the mass of the conducting layer. When electric radia- 

 tion is absorbed by the sensitive surface, there is a sudden diminution 

 of the resistance, and the galvanometer in circuit is violently de- 

 flected. By adjusting the electromotive force of the circuit the 

 sensitiveness of the receiver may be increased to any extent desir- 

 able. The receiver at each particular adjustment responds best to a 

 definite range of vibration lying within about an octave. The same 

 receiver could, however, be made to respond to a different range by 

 an appropriate change of the electromotive force acting on the circuit. 

 Very careful adjustment of the E.M.F. of the circuit is necessary to 

 make the receiver respond at its best to a particular range of electric 

 vibration. 



The Cylindrical Grating. The source of radiation the spark gap 

 being a line, the curved diffraction grating is made cylindrical. 

 The spark gap is always kept vertical ; the grating is made of equi- 

 distant metallic strips, which are vertical and parallel. A piece of thin 

 sheet ebonite is bent in the shape of a portion of a cylinder and kept 

 in that shape by screwing against upper and lower circular guide 

 pieces of wood. Against the concave side of the ebonite are stuck 

 strips of rather thick tinfoil at equal intervals. Five different 



o 2 



