1895.] 



of various Substances for the Electric Ray. 



163 



a radius of 8 cm. only. But when the cost of the material is not 

 prohibitive, it is advisable to use fairly large cylinders. The cylin- 

 ders used in the following experiments were 27*4* cm. in diameter, 

 and 10 cm. or more in height. For liquids, the cylindrical glass 

 trough used has a diameter of 25 cm. 



The tube of the radiator is fixed and points to the centre of the 

 graduated circle. The vertical central line of the cylinder passes 

 through the centre of the circular platform. 



The Receiver. — The receiver is a modified form of the coherer. In 

 a rectangular piece of ebonite a narrow groove is cut out. In this 

 groove bits of coiled steel springs are arranged side by side, only one 

 layer deep. In this way a linear receiver is constructed with a 

 sensitive surface 2 cm. in length and 4 mm. in breadth. By means 

 of a screw, the springs may be gradually compressed, reducing the 

 resistance. The coherer is in a circuit with an aperiodic D' Arson val 

 galvanometer and a copper-iron cell. The galvanometer has a 

 resistance of 300 ohms, and the voltaic cell has an E.M.F. of about 

 045 volt. A Daniell cell is sometimes used, with a resistance box as a 

 shunt; the E.M.F. may thus be adjusted to suit the sensitiveness of 

 the receiver. When the spiral spring coherer is freshly made, it is 

 over sensitive. On the second day it settles down to a fair condition, 

 though at first for about half an hour its action is rather unsteady. 

 But afterwards the sensitiveness becomes fairly uniform. It wilt 

 maintain this state under favourable conditions for nearly an hour ; 

 after which it begins to lose its sensitiveness. It must also be borne 

 in mind that the sparking balls are also undergoing deterioration. 

 The sensitiveness of the coherer may be partially restored by sub- 

 jecting it to electric radiation at close quarters, and slightly raising 

 the E.M.F. of the circuit. In this way it is sometimes possible to 

 work continuously for about two hours ; but greater weight should 

 of course be given to the first sets of observations, which are taken 

 at a time when the receiver is most sensitive. 



It is superfluous to add that special precautions should be taken to 

 guard against the disturbance due to stray radiations. The walls of 

 the room, the table, even the person of the experimenter himself may 

 act as reflectors, scattering the rays in all directions. I spent a con- 

 siderable time in trying to find a substance that will act as a good 

 absorber. Lamp black is useless, as it reflects copiously. Blotting- 

 paper soaked in water or copper sulphate solution does produce a 

 certain amount of absorption ; but even with these a certain amount 

 of reflection is found to take place. 



By proper screening, the disturbance due to stray radiations may, 

 however, be got rid of. The radiating apparatus, with the excep- 

 tion of a tubular opening, is completely enclosed in a metallic box. 

 The radiator tube extends right up to the refracting cyliuder. The 



