February i, 1889.] 



SCIENCE 



79 



ELECTRICAL NEWS. 



Hertz's Researches on Electrical Oscillations, i 



In reading Faraday's " Experimental Researches on Electricity," 

 one finds many experiments described in which that philosopher 

 attempted to show that some relation must exist between natural 

 phenomena which had been considered as having no connection 

 with one another. He tried to prove that gravitation could pro- 

 duce magnetic or electrical effects ; and although he failed, yet at 



„_5Z 



O ; — ^ — 



the end of the research he expressed his belief that some connec- 

 tion existed, and would some day be discovered. But he did find 

 that a magnetic field had an effect upon polarized light passing 

 through it, and this discovery has been developed until Maxwell 

 advanced the hypothesis that electrical vibrations may be propa- 

 gated- through space according to the same laws that govern the 

 propagation of light, and with the same velocity ; and now the 

 theory of Maxwell has been experimentally verified by Hertz. The 

 evidence in favor of Maxwell's theory, before Hertz published his 

 work, lay principally in the facts that the equations of propagation 

 of an electro-magnetic disturbance were practically those which 

 have been deduced for light, and which have explained a number 

 of the phenomena that take place. The velocity of such a disturb- 

 ance is equal to a constant which can be determined electrically, 

 and which agrees very well with the observed velocity of light; 

 while a relation between the specific inductive capacity, determined 

 electrically, and the index of refraction of a substance, agrees 

 fairly well wiih that deduced from Maxwell's theory. Hertz has 

 added to this evidence a proof — indirect, but fairly conclusive — 

 that the " electrical displacements " in a dielectric, on which rests 

 much of Maxwell's theory, really exist ; that actual electro-mag- 

 netic waves are reflected and interfere with one another ; and that 

 the velocity of such waves is about what is calculated. We will 

 describe Hertz's experiments as clearly and briefly as possible, re- 

 ferring those who desire a detailed description to Hertz's and De 

 Funzelmann's papers. 



In the first place,itwas necessary to produce' electrical oscilla- 

 tions of a very small period. Roughly, the period of alight-wave is 

 10 — "' of a second, and we know of noway of producing electrical 

 oscillations, on a conductor of finite size, of any thing near so rapid 

 a period. When Hertz took the matter up, the most rapid oscilla- 

 tions that had been experimented upon were those caused by the 

 discharge of a Leyden jar through a resistance, the period being 

 about 10 — ° of a second. Theoretically a much shorter period 

 would be obtained with conducting-wires forming an open circuit, 

 the ends having small knobs on them ; and this is the form Hertz 

 first experimented on, his object being to discover whether measur- 

 able oscillations were really produced. The period in this case 

 should be some hundred millionths of a second, — lo — ' as com- 

 pared with 10 — '° for light. The general arrangement of the ap- 

 paratus is shown in Fig. i. Here A is a large induction-coil, with 

 the wires B fastened to the terminals of the secondary circuit. The 



' Hertz's original papers are in Wiedemann's Annalen, 1888. Mr. G. W. de Funzel- 

 mann has given an excellent rc'sutne ol Hertz's work in the London Electrician. Nos. 

 539 545, 547, 548. 



coil Hertz used was a large one, 52 centimetres long by 20 centi- 

 metres in diameter, and it was run by six large Bunsen cells. 



When the coil is working, there are sparks between the knobs 

 B; and, when one of these sparks passes, we have the case of the 

 discharge of conducting-wires forming an open circuit ; and this 

 will cause rapid oscillations along the wires, which will diminish in 

 amplitude until they are re-enforced by another discharge. To 

 observe these disturbances, Hertz arranged a circuit, shown in 

 Fig. I at M. This was simply an open circuit of wire, with two 

 adjustable knobs, i and 2. On connecting this " micrometer " 

 circuit to one of the wires B, the connection being as shown in Fig. 

 I, sparks, sometimes several millimetres long, passed between i 

 and 2. 



The reason of this is, that the rapid and violent oscillations 

 on B are transmitted to the knob i, sometimes causing a high and 

 sometimes a low potential at that point. Now, if it takes a finite 

 tim.e for this disturbance to travel around the circuit M, then there 

 will be for a short time a considerable difference of potential be- 

 tween I and 2, and this causes the spark. With the connection to 

 M made as in Fig. 2, the distances of e from i and 2 being equal, 

 then the disturbance reaches these points at the same moment, and 

 there is no difference of potential between them, and therefore no 

 spark. This is what Hertz actually finds. 



Some of the facts that Hertz obtained were these : In the 

 first place, the effect in the micrometer circuit depends on the shape 

 of the terminals B and on the nature of the spark : the material 

 and size of the wire of the micrometer circuit has very little influ- 

 ence on the spark. If, when the contact is at e (Fig. 2), a con- 

 ductor be joined to one of the knobs i or 2, then the spark re-ap- 

 pears. 



This last. Hertz thinks, shows that the phenomenon cannot 

 be due to single waves in the directions ca and db respectively, 

 but must be due to a series of oscillations set up in the micrometer 

 circuit ; and the addition of a conductor to one of the knobs 

 changes the period of vibration of that branch, the periods being 

 determined by the product of the coefficients of self-induction into 

 the capacities of the branches. The fact that the material of which 

 the circuit is made has no effect on the spark, tends to show the 

 same thing ; namely, that the phenomena in the micrometer circuit 

 are dependent upon self-inductions and capacities, that is, upon 

 time-constants. 



f.L 



To further prove the oscillatory character of the discharge. Hertz 

 used the arrangement of Fig. 3. Here the micrometer circuit was 

 placed with one of its sides parallel to the wire zg/i. which was 

 connected to one of the discharge wires B. The sparks at .1/ were 

 very feeble until a conductor, C, was attached to the free end, /^, of 

 the wire, when they increased to two millimetres. No effect was 

 produced when C was attached at^. When the knobs at B were 

 so far apart that no sparking took place, the sparks at 31 also dis- 

 appeared, which showed that they were due to the sudden dis- 

 charges, not to ihe charging current. While C was attached to k. 



