the Study of the Properties of Electric Waves. 61 



electric radiation falls on the sensitive surface the spot is 

 deflected. By a slight unscrewing the resistance is in- 

 creased, and the spot made to return to its old position. The 

 receiver is thus re-sensitized for the next experiment. 



The sensitiveness of the receiver may be increased by a 

 proper adjustment of the E.M.F. acting on the receiving- 

 circuit. 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 

 E.M.F. ; very careful adjustment of this is necessary to 

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

 electric vibration. For simple experiments the adjustment of 

 the receiver is not difficult ; but for delicate experiments 

 careful manipulation is necessary. 



The proper adjustment of the E.M.F. is effected by taking 

 a derived current from a circular potentiometer-slide, fixed at 

 the base of the galvanometer. A simpler way is to take a 

 U-tube, the two limbs being respectively filled with copper- 

 sulphate solution and dilute sulphuric acid. Mixture of the 

 two solutions is prevented by an interposed plug of asbestos. 

 A rod of copper and a rod of zinc are plunged in the two 

 electrolytes, the whole forming a modified Daniell cell. The 

 cell is shunted by a suitable resistance, the receiving circuit 

 being connected to the ends of the shunt. The current 

 flowing through the shunt, and therefore the derived E.M.F. 

 from its ends, is varied by plunging the rods more or less in 

 the solutions. 



The leading wires from the ends of the receiver are 

 enclosed in layers of tin-foil ; the galvanometer and cell 

 have a metallic cover with a slit for the passage of the re- 

 flected spot of light. The receiving circuit is thus shielded 

 from the disturbing action due to stray radiations. 



The receiver is provided with a collecting funnel. This 

 prevents lateral waves from acting on the receiver. The 

 funnel has two hinged side- doors, by which its area — and, 

 therefore, the amount of radiation collected — may be varied. 

 When angular deviation is to be measured, the doors are 

 made parallel and perpendicular to the layer of spirals. The 

 aperture is reduced, and the receiver then only responds when 

 the funnel points to the direction of the deviated ray. 



In polarization experiments it is necessary to adjust the 

 receiver carrying the analyser in a crossed position. This is 

 done by a tangent screw, the rotation of the analyser being 

 measured by means of an index and a graduated vertical 

 disk. 



