Page:Collected Physical Papers.djvu/105

Rh By means of a very fine screw the upper sliding piece can be gently adjusted in or out. The spirals can thus be very gradually compressed, and the resistance of the receiver diminished. The galvanometer spot can thus easily be brought to any convenient position on the scale. When electric radiation falls on the sensitive surface the spot is suddenly deflected.

The receiver thus constructed is perfectly reliable; the sensibility can be widely varied to suit different experiments, and this sensibility can be maintained fairly uniform. The sensitiveness, when necessary, can be exalted to almost any extent, and it is thus possible to carry out some of the most delicate experiments (specially on polarisation) with certainty. In my more recent apparatus I use a single-contact receiver made of steel, nickel, aluminium or magnesium. These receivers can not only be made extremely sensitive, but also highly reliable.

The main difficulties having been thus removed, I attempted to construct a portable apparatus, with which all the experiments on electric radiation could be carried out with almost as great an ease and certainty as corresponding experiments on light, and with which even quantitative results could be obtained with fair accuracy.

The complete apparatus is here exhibited; all its different parts, including the galvanometer, and all the accessories for reflection, refraction, polarisation, and other experiments, are contained in a small case only 60 cm. in length, 30 cm. in height and 30 cm. in breadth. The apparatus can be set up and various adjustments made in the course of a few minutes.