Electric Waves along Wires by a Terminal Bridge. 41 



in the case of plane electric waves proceeding along parallel 

 leads to a bridge at the end of the line, the ratio of the 

 potential-differences of the two leads due respectively to the 

 reflected and incident waves is given by 



R— ~Lv 

 ' = KTW (1) 



where p is the ratio in question, or reflexion coefficient, R is the 

 resistance of the bridge, which must be of negligible induct- 

 ance, L is the inductance per unit length of the line, and v is 

 the velocity of light. Electromagnetic units are to be under- 

 stood throughout. 

 Thus, 



/>=-l, forR=0, (2) 



p= + l, forR=co, (3) 



and p= 0, for R=Lv (4) 



Now v = Sx 10 10 cm. per sec. nearly, and in the present case 



L = 41og e /——-j= 18'68 nearly. Hence the critical value 



of R to give complete absorption is, 



R=3 x 10 10 x 18*68 absolute units of resistance = 560 ohms. (5) 



Details of Bridge. — At first an ordinary resistance-box was 

 used as a bridge across the two wires of the line. But this 

 was soon rejected lest the faces of the two bars at a plug 

 opening should act as the plates of a condenser of appreciable 

 capacity, which is inadmissible in the bridge desired. We 

 next wound a small coil non-inductively on a bobbin in the 

 usual manner and used it without any box or screw terminals. 

 It was of well-covered wire and further insulated by thorough 

 soaking in paraffin-wax both before and after winding. But 

 this too proved unsatisfactory. For a time it stood, but then 

 the insulation broke down and sparks were seen to pass 

 between different turns of the wire. It was accordingly 

 abandoned. 



The simple expedient of making an electrical resistance by 

 rubbing a common lead pencil on a disk of ground glass, 

 small pieces of tin-foil gripped on by binding-screws forming 

 the terminals, was then tried, and seemed in every way 

 satisfactory. The electrostatic capacity of the terminals was 

 proved to be negligible by testing such a disk and screws 

 without any pencil. In this case no reflexion of the waves 

 could be detected. Moreover, by making the track of the 

 pencil-marks between the terminals very wide, the self- 

 induction of the bridge was reduced to a minimum. It 



