Electric Radiation Meter. 55 



silk allowing it freely to untwist as much as it would. After 

 this it was rolled between two hard surfaces and again pulled 

 out. This was repeated till at last 10 revolutions per millimetre 

 extension were obtained. 



The mirror was a y^-inch worked concave one, and gave 

 a very sharp image of a wire on a scale one metre off. Using 

 an ordinary galvanometer-scale, it was easy to read to one 

 division. This would correspond to an elongation of the wire 

 of '000005 of a millimetre, and a rise of temperature about 

 •003 of a degree Centigrade. In spite of this sensitiveness, 

 only one or two divisions' deflexion were obtained with the 

 oscillator 4 metres off. The oscillator used consisted of two 

 brass rods *53 cm. in diameter supported horizontally and 

 carrying two zinc plates 40 cm. square, capable of sliding along 

 them so that the wave-length could be altered at pleasure. 

 They were usually kept about 25 cm. apart. The terminal 

 knobs were 2 cm. in diameter and the spark-gap about 2 or 3 

 millim. It was noticed, that one of the knobs blackened very 

 quickly but remained quite cool, while the other, which altered 

 little, became very hot. 



The induction-coil employed to work it was 20 cm. long 

 and 12 cm. diameter, and gave with the battery-power em- 

 ployed sparks 4 cm. long between two points. To obtain 

 greater regularity in the working of the coil, a tuning-fork 

 mercury break, vibrating 86 times a second, was used. 



By employing a larger coil larger deflexions could, no 

 doubt, be obtained and effects at greater distances observed. 



A method was tried for increasing the sensitiveness by 

 weighting the mirror so that its centre of gravity was behind 

 and rather above the axis of suspension. The tension was 

 adjusted so that the spring kept the mirror near its position 

 of unstable equilibrium. The effect was to render the smaller 

 deflexions nearly 10 times as great, while the sensitiveness 

 diminished as the deflexions became larger. But the diffi- 

 culties of working with it were greatly increased, as the 

 smallest draught of air would alter the zero-point. 



"Wires of different materials were also tried which should 

 theoretically give better results ; but, probably owing to their 

 diameters not being sufficiently fine for the tension to which 

 they were subjected, the results were not satisfactory. The 

 convenience of being able to compensate the platinum wire by 

 the glass tube seems, however* a great point in favour of the 

 use of platinum, and has led me hitherto to keep to it. 







