Page:Collected Physical Papers.djvu/71

Rh however, occasional failures, owing either to the fault of the break, or loss of sensitiveness of the receiver.

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 not easy 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 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 $1⁄100$ 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. But by very careful and tedious adjustments I was however able 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