Phenomena Of Alternating Currents Of Very High Frequency

Date: 
Friday, May 22, 1891
Volume: 
3
Pages: 
649-649
Archived Page: 
Author: 
Subject: 

li-_ Mom e 1891.] m """°'“""° '°"“"” “I” 299 ' ELECTRICAL _ Review. be satisfied is of little practical value, owing to the small electromotive force obtainable und the difficult-ies in taking oil’ the current. With their keen inventor’s instinct, the now successful ure light men have early recognised the desideratums of a con- stent current machine. Their are light machines have weak fields, large ermatlues, with a great length of copper \vire and few commutator segments to produce great variations in the cnrrentfs strength and to bring self-induction into plny. Such machines may maintain within considerable limits of vnrin- tion in the resistance of the circuit e practically constant current. Their output is, of course, correspondingly di- minished, and, perhaps, with the obyect in view not to cut down the output too much, e simpe device compensating exceptional variations is employed. The undulntion of the current is almost essential to the commercial success of an are-light system. It introduces in the circuit s steadying element ts ing the place of e large ohmic resistance, with- out involving e great loss in power, end, what is more im- portant, it allows the use of simple clutch lamps, \vhich with H. current of e certain number of impulses per second, best suitable foreach particular lamp: will, if pro rly attended to, regulate even better than t e finest clocllivork lamps. This discovery hes been mnde by' the writer-several years too late. It has been asserted by competent English electricians that in a constant current machineor transformer the regulation is effected by varying the phase of the secondary current. That this view is erroneous may be easily proved by using, instead of lamps, devices, each possessing self-induction and capacity, or self-induction end resistance-that is, retarding and accelerating components-in unch proportions as tp not sHect materially the phase of the secondary current. Any number of such devices may be inserted or cnt out ; still it will be found that the regulation occurs, s constant current being maintained, while the electromotive force is varied with the number of the devices. The change of phase of the secondary c1u'rent is simply s result following from the changes in resistance, and, though secondary reaction is always of more or less importance, et the reel cause of the regulation lies in the existence of the conditions above enu- merated. It should be stated, however, that in the case of u machine the above remarks are to be restricted to the cases in which the machine is independently excited. If the exci- tation be eliected by commutating the armatnre current, then the fixed position of the brushes makes un shifting of the ncutml line of the utmost importance, und it muy not be thought immodest of the writer to mention that, ns fur us records go, he seems to have been the first who has succws- fully regulated machines in providing e bridge connection between e point of the external circuit and the commututor by means of n third brush. The ermuture und field being properly proportioned, und the brushes placed in their deter- mined positions, a constant current or constant potential re- sulted from the shifting of the diameter of commutation by the varying loads. In connection with machines of such high frequencies the condenser affords sn especially interesting study. It is easy to raise the electromotive force of such a machine to four or five times the value by simply connecting the condenser to the circuit, and the writer has continually used the con- denser forthe purposes of regulation as suggested by Blukesley in his book on alternate currents, in which he has trcntcd the most frequently occurring condenser problems \Vli¢l\ ex: uisite simplicity and cleurncss. The high frequency allows the use of small capacities und renders investigntion cusy. But( although in most of the experiments the result may be fore- told, yet some phenomena observed seem nt first cu1'ious. One experiment performed three or four months ugo \vith such a machine und u condenser muy serve as illustration. A machine wus used' giving about 20,000 ultcrnutiuns per sccend. Two bun: wires of about 20 feet long und two millimetres diameter, in close proximity to each other, were connected to the terminals of the muchinc on the one end, und to ucondenser on the other. A small transformer with- out nn iron core, of course, was used to bring the reuding Within the l'1\l\gB of n Curdew voltmeter by connecting the voltmeter to the secondary. On the terminals of the con- denser the eleetromotive force was about 120 volts, and from there inch by inch it grndunlly fell until on the tcrminuls of the machine it wus about 65 volts. It wus virtnully us though the condenser \V0l'(5 u generator, end ithe line and urmutnro circuit simply n resistance connected to it. The writer looked for u case of resonance, but' he was unable to augment the effect by varying the capacity very carefully and gradually or by changing the speed of the machine. A case of pure resonance he was unable to obtain. When a. con- denser was connected tb the terminals of the machine-the self-induction of the armature being first determined in the maximum und minimum position and the mean value taken-the capucit \vhich gave the highest electromotive force corresponded most nearly to that which just couuteructed the self-induction wit the existing frequency. If the capacity was increased or diminished, the eleetromotive force fell as expected. With frequencies as high as the above-mentioned; the con- denser eifects are of enormous importance. The condenser becomes a highly efficient npparatus capable of transferring considerable energy; The writer has thought that machines of high frequencies may find use at leust in cases when tmnsmission ut great dis- tunccs is not contemplnted. 'l‘he increase of the resistance may be reduced in the conductors and exalted in the devices \vhen heating effects are wanted ; transformers ma be mode of higher efficiency and greater outputs, and valuable results may be secured by menus of condensers. In using machines of high fleqnency the \vriter has been able to observe con- denser effects \vhich would have otherwise escaped his notice. He has been very much interested in the phenomenon observed on the Fcrmnti mein, which hus been so much spoken of. Opinions have been ex ressed by competent electricians, but up to the present all stdil seems to be conjecture. Undoubtedly in the views expressed the truth m\1st be contained, but, as the opinions differ, some must be erroneous. Upon seeing the dmgmm of M. Fermnti in the Elcclritfan of December 19th, the \vriter has formed his opinion of the effect. In the absence of all the necessary data he must content himself to express in words the process which, in his opinion, must undoubtedly occur. ’ '1 he condenser brings nbout two effects: (1) it changes the phases of the currents in the branches; (2) it changes the strength of the currents. As regards the change in phuse, the eifect of the condenser is to nccelerute the current in the secondnr ut Deptford, und to retard it in the primary at London. ’d‘he former has the effect of diminishing the self-induction in the Deptford prininry, und this means lower elcctronlotive force on the s ynnnno. 'l‘he returdution of the pri1nm~y nt London, us fer us merely the phase is conmrncd, has little or no cRect, since the phnso of tie current in the secondary in London is not nrbitnirily kept. - Now, the second effect of the condenser is to increase the current in both the branches, it is innnaterinl whether there is equality between the currents or not ; but it is neeesssr to point out, in order to see the importance of the Deptford step~np tmnsformer, that an increase of the current in both the branches produces opposite eifects. At Deptfoni it means further lowering of the e ectromotive fo1'ce at the primary, und nt London it means increase of the electromotive force at the secondary. Therefore all the things co-act to bring about the phenomenon observed. Such actions, at least, have been formed to take plnee under imilnr conditions. When the dynamo is connected directly to the main, one can see that no such action cun happen. The \vriter hn.s been particulurl interested in the sugges- tions und vic\vs expressed by Mr. gwinbnrne. Mr. Swinburne hus frequently honoured him by disngrceing \vith his views. Three years ngo, when thc writer, ngaiust the pievailing opinion of engineers, advanced an open ciieuit transformer, Mr. Swinburne wns the first to condemn it by stating in the E']E5[l'iUl'(IIL; “'l‘he (Tesla) transformer must be inefficient; it hus magnetic poles revolving, and hns thus an open mag- netic circnit." » 1‘wo yours Inter Mr. Swinburne becomes the champion of the open circuit trnnsfurlncr, und otfers to con- vert iim. But, [BIIWUIU mzzlzullur, el IUJS mulamur in I/lie. The writer cunnot believe in the urlnnture reaction theory ns expressed in Ilnlzzuf/~ie.s, though undoubtedly there is some truth in it. Mr. Swinburne’s interpretation, however, is so broad thnt it may mean anythinrf. , - Mr. Swinburne seems to haveioccn the first who has called nttention to tho hunting of the coudensers. The astonish- ment expressed nt Lhut by the ublest electrician is a striking

TEE TELEGBLPHIU JOURNAL LID 30° ELECTRICAL REVIEW EMM” 6' 1891 illustration of the desirability to execute experiments on a. large scale. To the scientific investigator, who deals with the minutest quantities, who observes the faintest effects, far more credit is due than to one who experiments with appa- ratus on an industrial scale ; and indeed history of ,science has recorded examples of marvellous skill, patience and keen- ness of observation. But however great the skill, and how- ever keen the observer‘s perception, it can only be of advan- tage to magnify an effect and thus facilitate its 'study. Had Faraday carried out but one of his experiments on dynamic induction on a large scale it would have resulted in an incal- culable benefit. ‘In the opinion of the writer the heating of the ccndensers is due to three distinct causes : first, leakage or conduction; second, imperfect elasticity in the dielectric, and, third, surging of the charges in the conductor. In many experiments he has been confronted with the problem of transferring the greatest possible amount of energy across a dielectric. For instance, he has made incan- descent larnps the ends of the filaments being completely sealed in glass, but attached to interior condenser coatings so that all the energy required had to be transferred across the glass with a condenser surface of no more than a few centi- metres s

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