SCIENCE. 



[Vol XIII. No. 314 



spheres, and having a break in the middle for the discharge spark; 

 the secondary being a circle of copper wire, broken by an air-space 

 which was capable of adjustment by means of a micrometer screw. 

 The two were adjusted until they were in unison, and the effect of 

 the primary on the secondary was observed for different positions 

 of the latter. There are in reality two electro-motive forces acting 

 upon the secondary, — one an electro-static force, due to the rapid 

 variation of the distribution of charge on the primary ; the other 

 an electro-magnetic effect, due to the current oscillating back and 

 forth on the primary wire. Now, it is a matter of very great im- 



portance to find the effects of these two. In the lower part of Fig. 

 6 (Fig. 8 in the paper) the lines mn represent different positions of 

 the secondary, which was vertical. The sparks in the secondary 

 disappeared when the air-space was in the horizontal plane passing 

 through the primary, and were a maximum for points at right 

 angles to these. The arrows give the resultant force, which does 

 not differ greatly from the electro-static distribution due to charges 

 on A and A' . 



When the secondary was horizontal, as in the upper part of Fig. 

 6, in position / there were two maxima of spark distance, when the 

 air-space was at a-^ and a\ ; in position // the maxima were at a, 

 and a\, the distance at a.^ being the greater ; in position /// there 

 was but one maximum, at a^, with a point of disappearance at a,, ; 

 at /K there was a maximum at a^, a minimum at a\ ; at V there 

 was a maximum at a^, a minimum at a'5. From the position /// 



to the position V the line aa' swung rapidly from a direction par- 

 allel to AA' to one perpendicular to it. 



Now, what all these experiments mean is this ; the electro-static 

 force is more important than the electro-magnetic within the dis- 

 tances at which observations were made, excepting in the last 

 cases, IV and V, where the electro-magnetic force comes in. But, 

 as it is of the greatest importance to find out what takes place at a 

 distance from the primary. Hertz extended his observations until 

 the secondary was as much as 14 metres from the primary. At a 

 distance of about 1.5 metres the maxima and minima became in- 

 distinct, but beyond this they were clearly defined again. From 

 his observations. Hertz plotted out the distribution of force in the 



room, the result being like Fig. 7 (Fig. 9 in the paper), where the 

 lines indicate the direction of the force, the stars representing the 

 points where the direction is indeterminate. We see that at dis- 

 tances beyond three metres the electro-motive force is everywhere 

 parallel to the primary, — that is, the electro-static effect is negli- 

 gible, — and we tind that the effect diminishes very much more 

 rapidly in the direction of the vibration than at right angles to it. 

 For less distances than one metre, the distribution of electro-motive 

 force is practically that of the electro-static force. 



There are two lines at all points of which the direction of the 

 electro-motive force is determinate, — the line in which the primary- 

 oscillation takes place, and a line at right angles to it. But there 

 are regions in which the electro-motive force becomes indeter- 

 minate : these form two rings around the primary, the projections 

 being the stars in the figure. Since the electro-motive force within 

 them acts very nearly equally in every direction, it must assume 

 different directions in succession, for of course it cannot act in dif- 

 ferent directions simultaneously. The observations, then, lead to 

 the conclusion that within these regions the magnitude of the 

 electro-motive force remains nearly constant, while its direction 

 varies through all the points of the compass during each oscillation.. 

 Dr. Hertz thinks the results very difficult of explanation, unless we 

 suppose the electro-static and electro-magnetic electro-motive 

 forces are propagated with unequal velocities, in which case we 

 have within the annular regions two electro-motive forces at right 

 angles, and differing in phase ; and as a consequence the resultant 

 will turn through all the points of the compass at each oscillation. 



One great value of the above series of experiments lies in the 

 fact that they enable us to put aside such theories as do not agree 

 with the observed results ; and, as there are a considerable num- 

 ber of theories, we are thus saved much confusion. 



The next subject which Hertz took up was the idea of displace- 

 ment currents in a dielectric, — an idea which underlies so much 

 of Maxwell's work. Briefiy, the assumption made by Maxwell is,, 

 that, if two conductors are charged positively and negatively re- 

 spectively, then in the dielectric between them there is a corre- 

 sponding displacement of electricity across any surface surrounding: 

 either. The displacement current lasts as long as the conductors- 

 are being charged, and has magnetic effects, just as a current in a. 

 conductor has. To show the existence of these displacement cur- 

 rents. Hertz arranged the experiment shown in Fig. 8 (Fig. 10 in 

 the paper). Here the primary circuit consisted of the two conduc- 

 tors A A' joined by a wire with the air-space for the discharge in 

 the middle. The secondary circuit was adjusted in unison with 

 the primary, and was placed in such a position that there was no 

 sparking. If, now, a conductor C be held near A A' , equilibrium 

 was disturbed, and sparks passed at/l On removing C, and ap- 

 proaching a dielectric, if no effect was observed, then the dielectric 

 would be shown to have no magnetic action, and Maxwell's theory 

 would fall to the ground. But on trying the experiment, a decided 

 effect was produced, thus proving that the dielectric exerted mag- 

 netic actions, and that MaxwelKs notion of displacement currents- 

 is in all probability correct. 



Electric Pl.a.nts in the Navy. — The report of the naval' 

 inspector of electric lighting, Lieut. R. B. Bradford, contains a 

 summary of the work performed under the supervision of this 

 office. After describing the installations on the " Trenton," the 

 " Omaha," the " New Hampshire," the "Atlanta," the "Boston," 

 the " Chicago," the " Yorktown," the "Baltimore," the "Charles- 

 ton," and the " Pensacola," the report goes on to say that search- 

 lights are at times very useful, but discretion must be exercised as 

 to when and how to use them. During the recent English evolu- 

 tionary squadron exercises, the search-lights of the blockading 

 squadron failed to detect the escaping ships of the enemy, which 

 had, of course, all lights out, and every thing visible carefully 

 colored a dead black. On the other hand, the enemy's ships which 

 were not trying to escape used their search-lights to blind the eyes 

 of the blockaders and interfere with the rays of their searchers. 

 Recent experiments in Russia indicate that it is not an easy matter 

 to disable a search-light with machine-guns and shoulder-rifles, on 

 account of the light blinding the eyes and interfering with the aim. 

 It is found in Germany, however, that if search-lights are placed 



