360 The Followers of Maxwell. 



this way shown to be finite and of the same order as the 

 velocity of light.* 



Later in 1888 Hertzf showed that electric waves in air are 

 reflected at the surface of a wall ; stationary waves may thus 

 be produced, and interference may be obtained between direct 

 and reflected beams travelling in the same direction. 



The theoretical analysis of the disturbance emitted by a 

 Hertzian radiator according to Maxwell's theory was given by 

 Hertz in the following year.J 



The effects of the radiator are chiefly determined by the 

 free electric charges which, alternately appearing at the two 

 sides, generate an electric field by their presence and a magnetic 

 field by their motion. In each oscillation, as the charges on 

 the poles of the radiator increase from zero, lines of electric 

 force, having their ends on these poles, move outwards into 

 the surrounding space. When the charges on the poles attain 

 their greatest values, the lines cease to issue outwards, and the 

 existing lines begin to retreat inwards towards the poles; but 

 the outer lines of force contract in such a way that their upper 

 and lower parts touch each other at some distance from the 

 radiator, and the remoter portion of each of these lines thus 

 takes the form of a loop ; and when the rest of the line of 

 force retreats inwards towards the radiator, this loop becomes 

 detached and is propagated outwards as radiation. In this 

 way the radiator emits a series of whirl-rings, which as they 

 move grow thinner and wider; at a distance, the disturbance 



* Hertz's experiments gave the value 45/28 for the ratio of the velocity of 

 electric waves in air to the velocity of electric waves conducted by the wires, and 

 2 x 10 10 cms. per sec. for the latter velocity. These numbers were afterwards 

 found to be open to objection: Poincare (Comptes Rendus, cxi (1890), p. 322) 

 showed that the period calculated by Hertz was V2 x the true period, which would 

 make the velocity of propagation in air equal to that of light x v'2. Ernst Lecher 

 (Wiener Berichte, May 8, 1890; Phil. Mag. xxx (1890), p. 128), experimenting 

 on the velocity of propagation of electric vibrations in wires, found instead of 

 Hertz's 2 x 10 10 cms. per sec., a value within two per cent, of the velocity of 

 light. E. Sarasin and L. De La Rive at Geneva (Archives des Sc. Phys. xxix (1893)) 

 finally proved that the velocities of propagation in air and along wires are equal. 



t Ann. d. Phys. xxxiv (1888), p. 610. Electric Waves (English edition), p. 124. 



J Ibid., xxxvi (1889), p. 1. Electric Waves (English edition), p. 137. 



