hertz's keseakches on electrical waves. 177 



ductors can be deteruiiued, and it therefore becomes po.ssible to com- 

 pareiii this mauuer the vibration periods of pLites, spheres, ellipsoids, &c. 

 In the experiments made by Dr. Hertz, nodes were very* distinctly 

 produced when the wire was severed at a distance of either 8 meters or 

 5.5 meters from the nnll i)oint of the base line. In the lirst case the 

 nodes occnrred at distan(;es from the null point of -0.2 meter, 2.3 

 meters, 5.1 meters, and 8 meters, and in the latter case at distances of 

 -0.1 meter, 2.8 meters, and 5.5 meters. It appears therefore that the 

 (half) wave-length in a free wire cannot differ much from 2.8 meters 

 The fact that the wave-lengths nearest to P were somewhat smaller was 

 to be expected from the intluence of the plates and of the curvature of 

 the wire. This wave-length, with a period of 1.1/ huudred-millionths of 

 a second, gives, 200,000 kilometers per second for the velocity of prop- 

 agation of electrical wav^es in wires. Fizeau and Gounelle (Poggen- 

 dorff's Annaleri, vol. Lxxx, p. 158, 1850) obtained for the velocity in 

 iron wires 100,000 kilometers per second, and 180,000 in copper wires. 

 W. Siemens (Poggendorff's Annalen, vol. CLVii, p. 300, 1876), by the 

 aid of Leydeu-jar discharges, obtained a velocity of from 200,000 to 

 200,000 kilometers per second in iron wires. Dr. Hertz's result is very 

 nearly the mean of these, from which we may conclude that the order, 

 at any rate, of the vibration period as calculated by him is correct. 

 The value obtained cannot be regarded, independently of its agreement 

 with experimental results otherwise obtained, as a fresh determination 

 of tke velocity, since it rests upon a theory which is open to doubt. 



Interference of the direct actions with those transmitted through the 

 icire. — If the square circuit B is placed at the null point in the second 

 principal position, with the air space at its highest point, it will be un- 

 affected by the waves in the wire, but the direct action when in this 

 position was found to produce sparks 2 millimeters iu length. B was 

 then turned about a vertical axis into the first principal position in 

 which there would be no direct action of the primary oscillation, but 

 the waves in the wire gave rise to sparks, and by bringing P near 

 enough to A, a sparking distance of 2 millimeters could be obtained. 

 In the intermediate positions sparks were produced iu both these ways, 

 and it would therefore be possible to get a difference of phase, such 

 that one should either increase or diminish the effect of the other. 

 Phenomena of this nature were, indeed, observed. When the i)lane of 

 B was in such a position that the normal drawn towards A A' was 

 directed away from that side of the primary conductor on which P was 

 {)laced, there was more sparking than even in the principal ])osition; 

 but if the normal were directed towards Pthe sparks disappeared, and 

 only re-appeared when the air space was made smaller. When the air 

 space was at the lowest point of 7?, the other conditions remaining the 

 H. Mis. 224 12 



