SPARE HALF HOURS.

Otago Witness, Issue 2006, 4 August 1892, Page 46

SPARE HALF HOURS.

By F. A. Joseph,

THE AMBER SPIRIT.

The ancients found that when a piece of amber was rubbed it displayed peculiar properties, which they attributed to a spirit that dwelt within the amber, and many strange legends cling round this belief. The spirit the Greeks called Electron. From the simple experiments with a piece of amber and pith balls, through a long series of experiments ever growing the more complex, we have the evolution of that seivice whic' bids fair to revolutionise the modes of our existence in this work-a-day world. We have come to recognise the marvellous potency of this natural force which we are only just learning to control ; but exactly what it is and how it operates is still a mystery. However in our day, when there are men who stand on a high plane of knowledge, we might reasonably enough hopethat electricity will sooner or later be compelled to reveal its identity and not much longer be looked upon as it was by the ancients. The difficalties in the way are great, but not greater than those that beset the path of the astronomer, who can now weigh the planets, measure the speed of the stars, and ttll the elements of which they are composed.

Some recent experiments originated by Professor Hertz, of Karlsruhe, and verified by other scientists, prove what was long expected — the close relationship between light, radiant heat, and electricity. In order to arrive at a clear understanding of the results of the experiments, we must lead up from what ia familiar to the more elaborate and difficult experiments of the physicist in his attempts to determine the nature of electricity. The wave theory of propagation is perhaps a clumsy one whereby to describe the movement of eound or light through space, but for the purpose of illustration it is good enough. Who has not watched the waves of the 6ea dash against some beetling cliff, rise high up, and then return seaward again ? Any one watching such waves must have noticed the strange effect of the receditg waves upon the advancing waves. At times when the crest of a receding wave meets the crest of one advancing, both crests combine to raise a bigger wave than usual. But at other times the crest of the receding wave appears in a peculiar fashion to pass under the crest of the advaDcing wave, and attain its maximum of height in the hollow between two waves. This has the effect of lowerirg the height of the advanciug waves. What we learn from this, is that wave added to wave in a certain way increases the disturbance, but in another way decreases it. The illustration given will hold good whether we apply \t to water, air transmitting sound, or the hypothetical ether transmitting light or elecricity. If a tuning fork of t a certain pitch be struck, and another fork of the same pitch interposed between it and a sounding board, w_e can reproduce the phenomena of the waves of the sea on the cliff. The sounding board reflects the sound given out by the tuning fork, and as is well known to those who have made any study of the subject, the sound waves proceeding from the first tuning fork will cause the second, if of the same note, to sound also. Now by moving the Bounding board closer to tbe stationary fork it will be found that there are points at which the force of the reflected waves is added to that of the advancing waves, and the sound given out by the supplementary tuning fork is augmented ; while at other points the waves fit into each other, and the sound is reduced in a perfeatly analogous manner to the waves of the sea referred to above.

I£ a red-hot ball of iron be placed in the focus of a concave reflector, and another similar reflector be placed at a certain distance from it, the reflected heat rays will be brought together at a point where so much heat is developed that a match will kindle. By varying the position of the second reflector the heat waves may be made to cross each other in such a fashion as to reinforce or neutralise each other, just as iv the case of the waves of water.

If a beam of light be thrown upon a plane surfaced mirror, and a similar mirror set opposite it at a certain angle, the advancing waves and receding waves may, by changing the relative positions of the mirrors, be made to reinforce or neutralise each other. In the former case we have increased light, and in the latter we may have absolute darkness. Thus we have to propound the startling theory that under certain conditions sound added to sound produces silence, and light added to light produces darkness 1

Professor Hertz's ingenious experiments demonstrate in a marvellous manner the homogeneity of light and electricity. The difficulties in t he way were greit, but one by one he overcame them till the problem lent itself to complete solution. Dealing with an imponderable and invisible force was more difficult than in the case of light", whose effects are easily scon. The first difficulty was togetareflector forthe eiec f iic waves, and the next was to produce waves of small enough length to deal with. If sound waves follow each other at the rate of 1000 vibrations iv a second, the speid of sound through dry air being 1100 ft per second, one wave will have travelled a little over a foot before it is followed by another. The wavps will therefore be 11-10 ft loDg. But if electrical discharges succeeded each other at the rate of 1000 a second, the electric waves would be each 180 miles loDg, since it has been demonstrated that electricity travels through space at the rate of about 180,000 miles a Eecond. Profes3or Hertz had to produce electrical discharges alternating 30,000,000 times a second to produce waves following each other at a distance of 35ft or 40ft. The reflecting board was made out o£ a sheet of zinc 10ft or 12£t square, and the detector, which takes the place of the tuning fork when sound waves are dealt ■with, consisted of a brass wire with two knobs bent into a ring so as to bring the knobs close together, The electric spark between the knobs showed when electrical waves struck the detector. The vibrator, by means of which the electric waves were discharged into space, consisted of two brass plates 12in square, to each of which was attached a thick wire about 2in long and terprinated by a brass knob. The distance

between the two knobs was very small, and when the brass plates were electrified by an induction coil, a series of sparks jerked from one knob to another. As soon as sparking began in the vibrator, the detector was approached to it, and sparks began to jerk between the knobs of the latter, By means of the reflector the electrical waves could be reinforced or the reverse. If the reflector was placed at a short distance from the detector, strong sparking ensued ; if removed to a distance of 18ft, the sparking ceased ; but when removed back still other 18ft, the sparking began agaiD, the two waves reinforcing each other just as in the casts of the tuning fork. The Jesuits of these experiments but prove the uniformity of nature, and the continuity of natural law. The speed of electricity has thus been proved to be identical with that of light — 180,000 miles per second — whether discharged through the air or sent along conductive wires. The experiments made with electro-magnetism were no less instructive. A plate of zinc sent the waves back, just as a mirror reflects light, but the electric waves could be sent through a wooden door into the next room as easily as light could be sent through a glass one, and the magneto-electric waves could be beat by a prism of pitch just as the waves of light are bent by a prism of glass. We learn from these experiments that electricitity, like light and radiant heat, is but the expression of a form of energy ; or, to speak more correctly, boSh are the result of disturbances of matter transmitted according to natuial law. When the waves of ether,' follow each other so rapidly that their length isonlyl2-millionths to 1G - millionths of an inc'i we have chemical energy — the so-called actinic rays beyond the visible spectrum at the blue end, and which shtfw themselves only on the photographic plate. When the waves follow each other at distances of from IG-millionths to 30-millionths of an inch, wo have the phenomena of light; when this waves grow in length to 12-100.000t.hs o1o 1 an inch, we see them no more, but feel them as radiant heat ; and when they have lengths of yards and miles they give out electrical ' phenomena 1 Thus we see continuity along the whole line — heat waves increasing in frequency and merging into light; light waves changing their rhythm and merging into electricity.