Polarisation of Electric Waves by a Twisted Structure. 147 



On the other hand, with small radiators, the difficulty is in the 

 proper adjustment of the receiver. It then becomes necessary to 

 have very exact adjustments of the receiver, both as regards the 

 pressure to which the sensitive spirals are subjected and the E.M.F. 

 acting on the circuit. It is only after some practice that the 

 peculiarity of each receiver is properly understood, when it becomes 

 easy to make the necessary adjustments by which the receiver 

 becomes quite certain in action. For various reasons the radiations 

 emitted by small radiators are more favourable for work requiring 

 great delicacy. 



In order that the surface of the radiator should be little affected 

 by the disintegrating action of the sparks, I use a single spark for 

 producing a flash of radiation. There used to be, however, some 

 uncertainty from a discharge occasionally failing to be oscillatory. 

 The cause of this uncertainty is ascribed to the deposit of dust on the 

 sparking surface. For greater certainty of action some observers 

 immerse the radiator in oil. The use of oil is under any circum- 

 stances troublesome. This is specially so in polarisation experiments, 

 when the radiator has to be placed in different azimuths. I have for 

 these reasons avoided the oil-immersion arrangement, and have tried 

 to secure certainty of oscillatory discharge without this expedient. 

 Attention was specially paid to the coil and the primary break. 

 A radiator has also been constructed which is found to be extremely 

 efficient. It consists of two platinum beads, each 2 mm. in diameter, 

 separated by 0*3 mm. spark-gap. There is no interposed third ball. 

 This radiator, though kept exposed for days without any protecting 

 cover, was yet found to give rise to a succession of effective 

 discharges without a single failure. I even went so far as to pour a 

 stream of dust on the radiator, in spite of which severe treatment, 

 the sparks were found to be quite effective in giving rise to electric 

 oscillation. 



The receiver, too, is perfectly certain in its action, and various 

 degrees of sensitiveness may be given to it. In the following 

 experiments, the sensitiveness had to be very greatly enhanced, and 

 this, as alluded to above, was secured by proper adjustments. The 

 secondary disturbances were got rid of by careful screening. But 

 one serious difficulty was encountered at the very outset, in the 

 failure of the polariser to produce complete polarisation. In my first 

 experiments on polarisation (the receiver then used not having been 

 very sensitive), polarisers made of wire gratings were found effective. 

 But in my later experiments with still more sensitive receivers, 

 I found that, owing probably to the want of strict parallelism of the 

 wires and the difficulty of exactly crossing the analyser and polariser, 

 it was impossible to produce total extinction of the field. I then 

 made a polariser and analyser by cutting parallel slits out of two 



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