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SCIENCE. 



[Vol. XIII. No. 3.6 



2.3 metres, 5.1 metres, and 8 metres ; and in another experiment. 

 — .1 metres, 2.8 metres, and 5.5 metres. The half wave-length, 

 then, cannot be far from 2.8 metres, which would give, taking the 

 period as .000000014 of a second, a velocity of propagation of 

 200,000 kilometres a second, — a result which lies between Fizeau 

 and Gounelle's values of 100,000 kilometres for iron and 180,000 

 kilometres for copper, and Siemens's results, which gave from 

 200,000 to 260,000 kilometres per second for the volocity in iron 

 wire. Another important point is, that, if the copper wire be re- 

 placed by one of different metal,. the nodal points remain fi.xed, — 

 thg-t is, the velocity does not change, — and the same is true when 

 the diameter of the wire is changed. This result is striking, 



especially as showing that at such short periods the magnetic 

 properties of iron wire have very little effect on the phenomenon. 



Now, it is evident that a secondary circuit, such as B, Fig. 9 

 (Fig. II in paper), is subjected to two actions, — the action of the 

 current in inn, and the direct action of the primary. If we obtain 

 for any position oi B a. certain result, due to the combined action, 

 and if, keeping the direction the same, we shift B along the wire a 

 distance corresponding to one wave-length, then, provided the 

 direct disturbance travelled with the same velocity as that along 

 the wire, we should have a result of the same character as at 

 first : if the actions were primarily in the same direction, they 

 would still be so ; if they at first opposed, they would still 

 oppose one another. If, however, the actions travelled with un- 



equal velocities, they would change their relative directions as the 

 secondary was shifted along the wire, — slowly if the velocities 

 were nearly alike, faster if they differed considerably. This is the 

 result which Hertz obtained ; the change of sign taking place, when 

 the observations were at a considerable distance from the primary, 

 in a distance of 7.5 metres, — a result which would give for the 

 velocity in air a value equal to that in the wire multiplied by the 

 ratio of 7.5 to 7.5 — 2.8, or 320,000 kilometres per second. Consider- 

 ing the very rough methods of experiment, this agrees very well 

 with the velocity of light, which is approximately 300,000 kilo- 

 metres per second. 



But in this experiment a very important fact was developed.- 

 While the change of sign took place in a distance of 7.5 metres at 

 points considerably removed from the primary, yet near the primary 

 the change of sign was in a much shorter distance. Now, we 

 have seen that near the primary the electro-static effects are of the 



greater importance, while at a distance from it the electro-dynamic 

 actions can alone be considered. It would seem, then, that electro- 

 static actions are propagated with greater velocity than electro- 

 magnetic. It is hard to say from Hertz's results whether the 

 velocity is infinite, since at a distance of 2.8 metres — the half- 

 length of a wave on the wire — the magnetic effects are already 

 of considerable importance. Still it seems fairly well proved that 

 static and magnetic actions are propagated with different velo- 

 cities, the latter being approximately that of light. 



After this experiment. Hertz attempted to obtain evidences of 

 the reflection of electro-magnetic waves from conducting surfaces. 

 To do this, he placed the apparatus in a room whose dimensions 

 were 15 metres in length, by 14 metres in width, by 6 metres high. 

 There was a row of iron columns in the room, which cut off part of 

 the available space, so that the width that could be utilized was 

 reduced to 8.5 metres. All of the gas-fixtures having been 

 removed, one wall of the room was prepared as a reflector by 

 hanging on it a sheet of zinc, which was carefully attached to the 

 gas-pipes and water-pipes. The primary circuit was arranged 

 in a vertical position ; the secondary was fitted so it could be 

 moved into any position, this being usually accomplished by hand, 

 although the body of the observer exerted a slight influence on the 

 results. On placing the secondary in different positions between 

 the primary circuit and the wall, results were obtained which are 

 shown in Fig. 10 (Fig. 12 in article). With the secondary in the 

 positions /, //, ///, and /V, at distances from the wall shown by 

 the scale, the strongest sparks were obtained when the air-space 

 was in the position shown. At V. VI, and VII the same sparks 

 occurred with the air-space at either side. This can be accounted 

 for by supposing the waves to be those shown in the figure, the 

 vibrations from the primary being reflected from the wall. Deter- 

 mining the length of the wave in this way, and assuming that they 

 travel with the velocity of light, we obtain a period of .0000000155 

 for the oscillations, instead of the .000000014 calculated from the 

 dimensions of the circuit. 



Such, in brief, are the most important of the results obtained by 

 Hertz. He has developed a new experimental method, which in 

 his own hands, and those of a number of physicists who are with- 

 out doubt working now on the subject, will greatly extend our 

 knowledge of what takes place in the vicinity of varying electric 

 currents, and which will modify our views on the nature of electric 

 actions. So far Hertz has shown that oscillations of short period 

 can be practically obtained and experimented upon, and he has 

 developed a method of investigation by means of a secondary 

 circuit of a period equal to that of the vibration. He has shown, 

 that, of the electro-static and electro-magnetic phenomena that 

 accompany an oscillation, the former is of great importance near 

 the oscillating current, but rapidly die away as the distance in- 

 creases. He has shown that dielectrics have magnetic effects when 

 placed near a conductor carrying an oscillating current, thus mak- 

 ing more than probable Maxwell's hypothesis of " electric displace- 

 ment." The velocity of a wave along a wire was determined, and 

 from this the velocity of the electro-magnetic and electro-static 

 waves in air were experimented upon. The former was found to 

 be approximately that of light ; the latter was much greater, but it 

 was not possible to determine whether it was infinite. Lastly, 

 electro- magnetic waves were reflected from a conducting surface, 

 and their wave-length determined by the interference of the direct 

 and reflected waves. 



Hertz's work has put Maxwell's electro-magnetic theory of light 

 on a firm basis, and has added experimental evidence to what was, 

 after all, only an hypothesis. 



Electricity and Light.— At a meeting of the Berlin Physi- 

 cal Society, Dec. 14, 18S8, Professor von Helmholtz, the president, 

 gave an account of a paper by Professor Hertz, which he had com- 

 municated the day before to the Berlin Academy. It contained a 

 description of further experiments on electro-dynamic waves, and 

 their analogy with waves of light. Weak induction discharges 

 between small metallic cylinders with rounded ends were employed, 

 and a similar apparatus for the detection of the electro-dynamic 

 waves. The action was not propagated more than 2 or 3 metres 

 through space ; when it fefl on a metallic surface, it was reflected, 

 interference phenomena were observed, and from these the length 



