Feb. 



1889] 



NATURE 



Ice Planed. 



Perhaps it will interest some of those who are investigating 

 the structure of different kinds of ice to know how blocks of it 

 may, with ease and certainty, be shaped into bars and plates of 

 any required dimensions. Some time ago I had occasion to 

 prepare some specimens for experimental purposes. At first 

 my success in working the ice into the required form was not 

 very great, for it cracked in all directions under the action of a 

 saw or chisel. After trying many devices, I at last resorted to 

 a joiner's plane ; a tool which may have been tried for the 

 purpose before, though I do not remember having seen its use 

 suggested. With it ice may be planed with greater ease than 

 wood, the shavings coming away in powder, and leaving the 

 ice with a clean, bright, sound surface. R. M. Deeley. 



39 Caversham Koad, Kentish Town, N.W., February 16. 



REPETITION OF HERTZ'S EXPERIMENTS, 

 AND DETERMINA TION OF THE DIRECTION 

 OF THE VI BRA TION OF LIGHT. 



SINCE last October, Prof. Fitzgerald and I have been 

 repeating some of Prof. Hertz's experiments, as 

 occasion allowed.; and it may not be without interest at the 

 present time to give a short account of our work. 



The first experiment tried was the interference of direct 

 electro-magnetic radiation with that reflected from a 

 metallic sheet. This experiment is analogous to that 

 known in optics as " Lloyd's experiment." 



The radiation was produced by disturbances caused in 

 the surrounding space by electrical oscillations in a con- 

 ductor. It was arranged in this wise. Two thin brass 

 plates, about 40 centimetres square, were suspended by 

 silk threads at about 60 centimetres apart, so as to be in 

 the same plane. Each plate carried a stiff wire furnished 

 at the end with a brass knob. The knobs were about 3 



millimetres apart, so that on electrifying one plate a spark 

 could easily pass to the other. This spark, as is well known, 

 consists not simply of a transference of half the electricity 

 of the first plate to the second — though this, which is 

 the final state, is all that is observable by ordinary experi- 

 mental methods — but the whole charge passes across to 

 the second plate, then returns, and so on, pendulum- 

 fashion, the moving part of the charge becoming less each 

 time, till finally brought to rest, the energy set free at 

 sparking being converted partly into heat in the wire and 

 air break, partly into radiation into space, or in terms of 

 action at a distance in inducing currents in other bodies. 



The time taken by the charge to pass over to the 

 second plate and to return, is a definite thing for a given 

 sized arrangement, and depends on the connection be- 

 tween them. If C be the capacity of the plates, and I 

 the self-induction of the connection, the time of each 

 complete oscillation equals itt ,^0.1. The time in the 

 case of the particular arrangement used is, speaking 

 roughly, about the 1/30,000,000 of a second. 



If there be conductors in the neighbourhood of this 

 " vibrator," currents will as usual be induced in each on 

 every passage of the charge between the plates, each 

 passage serving simply as a primary current. 



Now, speaking briefly, the whole object of the experi- 

 ment is to find out if these induced currents take place 

 simultaneously in conductors situated at various distances 

 from the primary current, and if not, to determine the 

 delay. In order to do this we must, in the first place, be 

 possessed of some means of even ascertaining that these 

 currents occur, all ordinary methods being inadequate for 

 detecting currents lasting only for such exceedingly short 



periods as these do. By devising how to determine the 

 existence of these currents, Hertz made the experiment 

 possible. 



His method depends on the principle of resonance, 

 previously suggested by Fitzgerald, and his current- 

 observing apparatus is simply a conductor, generally a 

 wire bent into an unclosed circle, which is of such a 

 length that if a current be induced in it by a passage of 

 a charge across the " vibrator " the return current or rush 

 back of the electricity thus produced in the ends of the 

 wire occurs simultaneously with the next impulse, due to 

 the passage back across the " vibrator." 



In this way the current in the "resonator" increases 

 every time, so that at last the end charges, which are 

 always of opposite sign, grow to be so great that sparks 

 will actually occur if the ends of the wire are brought 

 near together. Thus, Hertz surmounted the difficulty 

 previously experienced by Fitzgerald when proposing 

 electro-magnetic interference experiments. 



The time of vibration in this circle is, as before, 27r a/CI, 

 but on account of difficulties in calculating these quanti- 

 ties themselves, the length of the wire is most readily 

 found by trial. To suit the " vibrator " we used, it was 

 about 210 centimetres of wire No. 17. The ends of the 

 wire were furnished with small brass knobs, which could 

 be adjusted, as to distance between them, by a screw 

 arrangement, the whole being mounted on a cross of wood 

 for convenience in carrying about. 



At first sight the simplest " resonator " to adopt would 

 seem to be two more plates arranged similarly to the 

 " vibrator," but it will be seen on consideration that it 

 would not do, because no break for seeing the sparking 

 could be put between the plates, for, if it were, the first 

 induced current would be too feeble to jump the break, 

 so that the reinforcement stage could never begin.^ 



The charging of the " vibrator " was effected by con- 

 necting the terminals of an induction-coil with the plates. 

 In this way a continuous shower of sparks could be 

 obtained in the resonating circle. 



The circle in the interference experiment was held in 

 the horizontal plane containing the axis of the " vibrator," 

 the ends of the circle of wire being in such a position that 

 a line joining the knobs was at right angles to the 

 " vibrator." In this position only the magnetic part of 



the disturbance could affect the circle, the "magnetic 

 lines of force," which are concentric circles about the axis 

 of the " vibrator," passing through the " resonator "circles. 

 When the knobs of the circle are brought round 

 through 90°, so as to be parallel to the " vibrator," the 

 electric part of the disturbance comes into play, the 

 electric lines of force being, on the whole, parallel to the 

 axis of the "vibrator." The electric action alone can 

 cause a forced vibration in the knobs, even when the 

 connecting wire is removed, if placed fairly close to the 

 " vibrator." 



* However, two pairs arranged in line, the pairs connected by a wire, 

 could probably be got to spark between the centre plates. 



