Short Electric Waves by Air and Water- Vapour. 707 



forming the linear receiver ; j, thermoelectric junction ; 

 <7, wooden rod supporting the receiver ; m, cross-section of 

 parabolic mirror ; d, wooden support ; and 11 copper wires 

 leading to the galvanometer. The wires a and I are con- 

 tinuous, being made of one and the same piece of wire. 



As already stated, each receiver was connected to one of 

 the two coils, G x and G 2 (fig. 1), of the galvanometer, in 

 such a manner that the thermoelectric currents passing- 

 through the coils produced nearly equal and opposite effects 

 upon the needle of the galvanometer when the air in both 

 tubes was at atmospheric pressure. 



The tubes T 1 and T 2 were placed so that their axes were 

 horizontal ; and the sender, S, and the receivers, R x and R 2 , 

 were placed vertically and approximately in the focal lines 

 of the cylindrical parabolic mirrors, P ? P x , and P 2 respectively . 

 The mirror P had a focal distance of about 3*7 cm. and an 

 aperture of 24 cm. : P x and P 2 were of course alike, and 

 each had a focal distance of about 3*0 cm. and an aperture 

 of 13 cm. The mirrors were made of sheet zinc, and each 

 was 20 cm. high. Each receiver was enclosed in a small 

 box made of asbestos paper to protect it from changes of 

 temperature. 



I (fig. 1) is the induction-coil used to charge the sender, 

 M is a rotating mercury interrupter, and t is a small glass 

 tube leading from T 2 to a Gaede rotating mercury pump. 

 The pressure within T 2 was read by one of three manometers : 

 for high exhaustions by means of a MacLeod gauge ; for 

 low exhaustions by means either of an open mercury mano- 

 meter or of a closed U-shaped manometer. T l was not 

 hermetically sealed, and therefore the air in it was always 

 at atmospheric pressure. 



The experiments were carried out as follows. The tube 

 T 2 was filled either with air or with water-vapour. In some 

 cases radium bromide in open vessels was placed within the 

 tube. It was then slowly exhausted and readings taken of 

 the pressure for every few centimetres, millimetres, or 

 fractions of a millimetre. 



Long series of readings were made, first with air alone, 

 then with water-vapour alone, and then with air in the 

 presence of radium. 



When experimenting with water-vapour, the air was 

 pumped out of the tube until what was left had a pressure of 

 •0017 cm. Water-vapour was then introduced into the tube 

 until the pressure had risen to 1'50 cm. The tube was then 

 again gradually exhausted, and-'readings of the deflexions 

 taken for decreasing pressures. 



