electric radiation falls on the sensitive surface the spot is deflected. By a slight unscrewing the resistance is increased, 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 reflected 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.
