704 



Prof. J. E. Ives on the Absorption of 



two tubes were placed side by side on a carriage to which a 

 horizontal motion could be given ; first one tube being- 

 placed between the sender and receiver, and then the other. 



The difficulty with this method is that it is impossible to 

 get waves of the same strength sent out from the sender 

 twice in succession. As the absorption, if there is any, must 

 be very small, it is necessary, when using this method, that 

 the strength of the waves should be the same, or very nearly 

 the same, each time. As is well known, when using an 

 oscillator actuated by a spark-gap, it is impossible to attain 

 this condition on account of the irregular action of the spark. 



The only way to overcome this difficulty appeared to be to 

 use a differential method. 



In the experiments to be described in this paper, such a 

 method was adopted, the waves being sent through both 

 tubes at the same time from one and the same sender, and 

 received on two separate receivers, one placed at the farther 

 end of each tube. Each receiver contained a thermo-electric 

 junction, and each junction was connected to one of the coils 

 of a two-coil Siemens and Halske armoured galvanometer * 

 in such a manner that the thermoelectric currents produced 

 by the electric waves acted oppositely on the needle. By 

 changing the position of the sender or of one of the receivers, 

 or by partially covering one end of one of the tubes by a 

 sheet of metal, the two disturbances of the galvanometer 

 could be made to nearly neutralize each other, so that only a 

 very small deflexion was obtained. 



The arrangement of the apparatus is shown, diagram- 

 matically, in fig. 1. 1\ and T 2 are two glass tubes both 



Fis:. 1. 



Showing arrangement of apparatus. 



exactly alike, 44*5 cm. long, and 10 cm. in diameter, and 

 having pieces of plate glass sealed, with sealing-wax, upon 

 their ends. The tubes were covered with tinfoil, so that the 



* This instrument is of the type designed by Du Bois and Rubens, and 

 is described in the Zeits.f. Instrumcntcnkundc, Jahrg. 1900, p. 65. 



