More serious is the difficulty in connection with the receiver. With the improvements adopted there is no difficulty, under any circumstances, to make the receiver very highly sensitive; but it is extremely difficult to maintain the sensitiveness absolutely uniform. I have in my previous papers explained how the sensitiveness of the receiver depended on the pressure to which the spirals were subjected, and on the E.M.F. acting on the circuit; and how the loss of sensitiveness due to fatigue was counteracted by slightly increasing the E.M.F. For each receiver there is a certain pressure, and a corresponding E.M.F., at which for a given radiation the receiver is sensitive. Having obtained these conditions, the sensitiveness can be increased or decreased to almost any extent by a slight variation of either the pressure or the E.M.F. An increase of pressure produced by the advance of the micrometer press screw through a fraction of a millimetre would sometimes double the sensitiveness; similarly an increase of E.M.F. of even 1100 volt increases the sensitiveness to a considerable extent.
The nature of the difficulties in maintaining the sensitiveness of the receiver uniform will be understood from what has been said above. These difficulties are indeed great, and appear at first to be insuperable. But by very careful and tedious adjustments I was able on several occasions to obtain fairly satisfactory results, and was in hopes of ultimately obtaining symmetrical values from the galvanometer deflections. The setting-in of the rainy weather has unfortunately introduced other conditions unfavourable to the maintenance of uniformity of the sensitiveness of the receiver. Owing to the excessive damp and heat the spirals get rusty in a short time, and variation in the sensibility is produced by the altered condition of the surface of the sensitive layer. The results of certain experiments I have carried out lead me to hope that this difficulty will, to a certain extent, be removed by covering the sensitive surface with a less oxidisable coating.
The deflections produced in the galvanometer can only be taken approximately proportional to the intensity of the absorbed radiation. It would be better to observe the diminution of the resistance produced by the incident radiation. This may be done with the help of a differential galvanometer and a balancing resistance.
G is a high resistance differential galvanometer, with two sets of electrodes, A, B; C, D; one pair of electrodes is in series with the receiver, and the other with a resistance box. When the receiver is adjusted to respond to the electric radiation, a weak current flows through it. The same E.M.F. acts on both the circuits. The compensating current, produced by a proper adjustment of the resistance of the box, brings the spot of light back to zero. The resistance of the box is equal or proportional to the resistance of the receiver.
