Air-space on Total Reflection of Electric Radiation. 307 



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 ; bnt ib is 

 extremely difficult to maintain the sensitiveness absolutely uniform. 

 T 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 sensitive- 

 ness 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 in- 

 creased or decreased to almost any extent by a slight variation 

 of either the pressure or the E.M.F. An increase of pressure pro- 

 duced 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 ywu v0 ^ increases the sen- 

 sitiveness 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 main- 

 tenance 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 sensi- 

 tive surface with a less oxidisable coating. 



The deflections produced in the galvanometer can only be taken 

 approximately proportional to the intensity of the absorbed radia- 

 tion. 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 com- 

 pensating 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. 



