November 12, 1891] 



NATURE 



43 



nodes looked for, because the distances between them remained 

 unaffected on changing the rate of alternation. The distances 

 from node to node also were found to measure different amounts 

 (though on the whole there was a decided tendency towards 

 regularity). The average distance apart of the nodes in the 

 different rings tried lay between lo and i8 inches. 



The occurrence of the nodes might have been very well 

 attributed to the ring being locally irregular in its susceptibility 

 to induction, but for the irreconcilable fact that the effects on 

 either side of a node were found to be of opposite phase, just 

 as it would be, were the phenomenon due to stationary inter- 

 ference waves. 



This was ascertained by means of two coils connected in the 

 same sense in series with the telephone. When these coils 

 were arranged at places of equal intensity, one on each side of 

 a node, no sound was to be heard in the telephone, the effects 

 neutralizing one another. A commutator, to throw in the coils 

 singly or together as desired, is convenient for making this 

 experiment. 



From this, one would naturally assume that the currents 

 induced on either side of a node must be of opposite sign, seeing 

 that they neutralize each other in the telephone ; but experi- 

 ments with the galvanometer show it not to be the case. To 

 test this, the galvanometer is connected up through a com- 

 mutator arrangement fixed to the originator of the primary 

 current in such a way as only to admit of the currents induced 

 in one direction passing. Tried in this way, no difference in 

 the direction of the current on either side of a telephone node was 

 found, or, indeed, any trace of a minimum effect at these points. 

 The thing can also be tested by means of a ballistic galvano- 

 meter, and a reversing key with battery, for, with a reversing 

 key and telephone, the nodes, which are quite independent of 

 the speed, are to be found, as well as the opposite phase effect. 

 The ballistic galvanometer gives no indication of there being 

 any difference at the nodes from elsewhere, and the deflection 

 everywhere is in the same direction. 



It was thought that perhaps the telephone effect was in some 

 way connected with the fact that the form of the alternating cur- 

 rent was not a simple wave or sign curve, owing to the method em- 

 ployed in producing it. This consisted of a rotating commutator, 

 which threw in circuit alternately two cells connected up singly and 

 in opposite directions. For this reason, the effect, when using a 

 small alternating machine with about 40 alternations per second, 

 was compared, and was found to be in no way different. Also 

 what must have been a very regular variable current of the 

 simple harmonic type was procured by means of a microphone 

 and an organ-pipe. This gave like results. 



One is thus left apparently to suppose the sound in the 

 telephone to be due to a peculiarity in the character of the 

 curve representing the rise and fall of the current, probably 

 something of the nature of a subsidiary oscillation ; this sub- 

 sidiary oscillation being absent at the nodes, and of opposite 

 sign on either side. 



As mentioned before, it is necessary for the alternating coil to 

 be placed at definite positions, in order that the system of 

 nodes and internodes should occur. These positions of the 

 alternating coil are at about the same average distance apart, 

 and are of very much the same character with respect to regu- 

 larity as the nodes of the telephone coil. In fact, if the alter- 

 nating coil and the telephone coil change places round the ring, 

 the best position for the alternating coil will always be between 

 two nodes, and the nodes will be found situated between two old 

 positions of the alternating coil. If the alternating coil be 

 placed at a point where a node was found in some other position 

 of the alternating coil, the system of nodes and internodes 

 generally completely disappears, and now on moving the tele- 

 phone coil round the ring the intensity uniformly diminishes 

 until the diameter is reached, and then increases round the other 

 half of the ring. This gives the phenomenon a distinctly 

 resonant character. The induced current, as observed by a 

 galvanometer, is always of the latter character— that is to say, 

 a uniform fall, and then a rise on going round the ring. 



As a rule the permanent magnetism of these large rings is 

 irregular, and apparently apt to change frequently. A determi- 

 nation of the permanent magnetism was easily made by means 

 of one of the coils connected with a ballistic galvanometer. By 

 moving this through a given amount at a time, say an inch, and 

 noting the throw of the needle, one was able to plot out a re- 

 presentation of the state of the permanent magnetism. In this 

 way, places where no throw occurs were found, while to either 



side of such a point the throw changed sign. It was sometimes 

 found that there was a decided tendency for the position of no 

 throw to occur between two telephone nodes, the throw changing 

 sign on either side of these points. But further experiments 

 showed that this arrangement of the permanent magnetism was 

 probably accidental, and due to the very currents employed in 

 making the telephone observations. For when only very feeble 

 currents had been used on a ring, these consequent poles weie 

 absent. 



It is possible, as one would expect, to artificially make a 

 minimum intensity position, at any point on a ring, by winding 

 on a few turns of thick copper wire. But the fact that the phases 

 on either side of such a point (found as before by means of two 

 coils in circuit with a telephone) are the same, precludes the 

 idea that the nodes can be due to Foucault currents. 



Obviously, however, the phenomenon depends on some 

 permanent peculiarity round the^ring which happens to occur 

 fairly regularly. What this peculiarity is, or how it is brought 

 about, I have not yet been able to discover. 



Fred. T. Trouton. 



OYSTERS AT THE ANTIPODES. 



CO much attention has been given in England to the various 

 •^^ questions connected with oyster-fisheries that it may be of 

 interest to note some facts relating to the oyster-fisheries of our 

 Australian kinsfolk. The subject was admirably dealt with in 

 a lecture delivered by Mr. Saville-Kent before the Christchurcl* 

 meeting of the Australasian Association for the Advancemenr 

 of Science. This lecture is entitled " Oysters and Oyster- 

 Culture in Australasia," and has been published separately. 



Mr. Saville-Kent devotes attention chiefly to Australia and 

 Tasmania, as, at the time when his lecture was prepared, he 

 had not had an opportunity of personally studying the question 

 in New Zealand. Beginning with Tasmania, where for five 

 years he was officially connected with the oyster- fisheries, he 

 points out that the oyster of Tasmania corresponds closely with 

 the type Ostrea edulis, produced and cultivated in British waters. 

 Formerly, this oyster was so abundant in Tasmanian waters, 

 that, according to the report of a Royal Commission of Fisheries 

 in 1882, about twenty years previously a quantity representing 

 at current prices a retail value of no less than ;^90,ooo had 

 been exported in a single year to Victoria and New South 

 Wale?. At that time, oysters were so plentiful that it was a 

 common practice to burn them in large quantities for the purpose 

 of making lime. The strain was, of course, too severe, and 

 by and by the Tasmanians found that, although there was still 

 a demand for oysters, there was no longer a home-supply, and 

 that it was necessary for them to go elsewhere for the commodity 

 which they had so recklessly wasted. In 1884, when Mr. Saville- 

 Kent reached the colony, the oyster-fisheries of Tasmania had 

 for some years been an obsolete industry. Profiting by the 

 information which had been made accessible through the Inter- 

 national Fisheries Exhibition and associated Conferences in 

 London in 1883, and by Prof. Hubrecht's testimony as to the 

 oyster-fisheries of the Schelde, Mr. Saville-Kent recommended 

 the establishment, in suitable localities, of efficiently-protected 

 Government reserves, upon which breeding-stocks of oysters of 

 the best quality should be carefully cultivated and permanently 

 retained. These reserves were to fulfil the double purpose of 

 breeding-centres, from whence the surrounding waters might be 

 restocked, and also of model oyster-farms, around which private 

 beds might be established on similar lines. The scheme recom 

 mended being approved, sites formerly associated with the most 

 prolific oyster production were selected. The operations were 

 necessarily conducted on a very modest scale. Oyster stock, 

 suitable for laying on the reserves, could be accumulated only 

 by slow and laborious processes, and some 20,000 to 50,000 

 oysters represented the approximate numbers that were gradu 

 ally collected and placed under cultivation. In order that the 

 largest possible amount of spat produced by the oyster stocks 

 laid down might be caught, various methods were adopted, the 

 principle being that which has been followed with so much 

 success by M. Coste on the west coast of France. In addition to 

 dead oyster-shells, or "cultch," which has, from the earliest days 

 of oyster-culture, been recognized as representing a most natural 

 and prolific catchment material for the adhesion of the spat, 

 artificial collectors of various descriptions were introduced. In 

 France, tiles cemented on their lower surfaces have been found 



NO. II 50, VOL. 45] 



