being inserted, so that when the globe, G, is put into place and the cage, C, screwed hard up, the whole fitting is thoroughly watertight. Fig. 9 is a roof light. Figs. 10 and 11 show a section of a sconce and a complete view. The connection is made by the three spring plungers, SP. The front has a hinge on the right side Others may be added, or those mentioned may be made more ornamental according to the fancy of the owner. In conclusion, I may say that people are sometimes inclined to expect an electric light installation to look after itself. This is expecting too much, and no installation, no matter how well it is designed and carried out, will give satisfaction if it is not properly looked after. BRITISH ASSOCIATION CARDIFF MEETING. FIG. 12,-Hand Lamp. V FIG. 13.-Gangway Lamp. and a screw on the left. WR is a white enamel, or mirror, reflector, round the edge of which a rubber washer, R W, of square section is placed in a suitable groove. When the lamp has been fixed and the front is closed, the shoulder on it, S, bears on the rubber washer, and when the screw, s, is entered and screwed up tight no water can possibly get in. Figs. 12 and 13 are hand lamps and gangway lamps, and Fig. 14 is a section of a cargo reflector. It consists of a metal shade, to which the brass bowl reflector is attached, the whole, together with the stock, forming part of the return. Inside is a metal rim, insulated from the shade, and from this an insulated wire is led to the centre contact in the gooseneck. Round the bowl, holes are bored and tapped to receive the lampholders, being so placed that when the latter are screwed home their contacts bear on the insulated rim. The fixing of the holders is thus rendered very simple, and a great deal of time is saved. FIG. 16.-Junction Sleeve. The flexible lead may be either twin wire or concentric wire. To each end of it a concentric swivel is soldered as shown in Fig. 15, and by screwing this into the gooseneck, and into a junction-box provided for the purpose, connection can be made in a few moments. By means of brass sleeves Fig. 16, any number of these leads can be joined into one, which in many cases will be found very convenient. This concludes the fittings which may be considered necessities. JOINT DISCUSSION ON UNITS BY SECTIONS A AND G. The name secohm DR. OLIVER LODGE: There are a good many things to be said about units at the present time, both mechanical and electrical. I think it will be for the convenience of the section if at the beginning of the discussion we attend principally to the electrical units, and to those units which are of established interest to engineers, and leave the more academical and mechanical units to a later period. With regard to electrical units, one of the most pressing things is the determination of an authoritative statement or agreement as to the coefficient of induction-not only selfinduction, but also mutual induction. Now, we know that already a great many names have been suggested for this unit. The mere name of the unit is, of course, very unimportant from some points of view, but it is not wholly unimportant whether this unit shall be called secohm or quadrant, or by the name of some man. Henry, mac, and other names have been suggested. has certain advantages, in that it indicates the relation between this unit and the ohm-second, but there are certain disadvantages in that it tends to express the more simple by the more complicated. It expresses the unit self-inductance as being derived from the ohm, whereas the determination of the ohm itself is based upon a measurement of inductance; instead of describing the self-inductance unit as an ohm multiplied by a second, it would be more logical to call the unit of resistance a self-inductance divided by time. In all methods of determining the ohm, some form of coefficient of induction is the length that enters into the expression. If it is to be called a quadrant, it must be remembered that a quadrant is strictly an angular measure; and even though it be understood as short for earth-quadrant, and therefore a length, still it is not unobjectionable, for it is pretending a thing to be a length when it really is not a length. I think it would be a backward step if we fix it for all time as a length. On the whole I should suppose that some name of less obvious meaning might be desirable, such as the name of a man. Now directly we permit ourselves to attached to the unit, we have the field open to consider. regard the name of a man as a possible name to be We are no longer compelled to make it equal to a secohm it may be a sub-multiple. Now let us consider whether the secohm is a convenient size. The C.G.S. units were a connected system, or might have been a connected system, before any practical units were invented. Practical units were invented for this reason, that C.G.S. units were not a reasonable size-they were a million times too big or too small. It was out of the question for engineers to be always speaking of powers of ten. Now a secohm is rather big for a practical unit. Very few people -I speak under correction-have to deal with coefficients of induction of more than a secohm. The question is whether it would not be better, therefore, to make our practical unit some sub-multiple, say a thousandth, or something of that sort. We have a consistent system in the C.G.S.; there does not seem adequate need for two consistent systems. I regard it as more important to have the practical units of convenient size, than to have them most simply related among themselves. The farad has been useless, by reason of the neglect of this idea. The whole object of a practical system is to have units of convenient size, otherwise the C.G.S. would be all that is necessary. : The next unit wanted is a practical unit of magnetic field, or the unit of magnetic induction. Let me just explain what I mean by a practical unit of magnetic field. There is a C.G.S. unit-and it has not at present had a name given to it, although sometimes names are used-but it is too small for engineering purposes; it is something comparable to the intensity of the earth's magnetism. The total intensity of the earth's magnetic field is about half of one of these units. The magnetic fields with which engineers have to do are usually enormously greater, hundreds or thousands of times. Moreover, the definition of C.G.S. magnetic field is not one which immediately applies to engineering practice. We do not deal with a unit magnetic pole; and although it serves fairly as a fundamental unit for theoretic purposes, it is not very practical. Now consider how magnetic fields do come in; they come into engineering practice in the case when you have a coil of wire rotating in such a field. You have a certain area through which a number of lines of force pass; each complete revolution of an armature coil cuts the total lines of force four times, and the voltage which you can get is just equal to the rate of cutting those lines of force. That is to say, if N represent the total number of lines of force passing through a coil, what would be called total magnetic induction through the coil, and if this gives the voltage; of which the value at any instant is the rate of the change of the magnetic induction. That is the fundamental formula of magneto-electricity. It contains almost the whole philosophy of the dynamo. Here is another formula which contains the whole philosophy of motors. d w = Cd N. The work done during any operation is equal to the current multiplied again by the change in the magnetic induction, the power being PeC. It would seem as well to have some practical name for this quantity, N, so constantly occurring, the total lines of force, or the magnetic induction. As for magnetic field, that is related to this quantity in the simplest possible way; for it is the number of lines of force through unit area. Whether the name shall be given to intensity of field or to magnetic induction-there might be a name for boththe relation between them is quite simple. The important thing to point out in most cases in practice is, that N is related to the coefficient of self-induction in a very simple manner. Suppose you have a closed solenoid, sectional area A, length 1, permeability , number of turns n, the total magnetic induction caused by a current, C, in (4πμη Α) N that solenoid is: = A) C It merely requires a slight modification to make it applicable to dynamo magnets and other things. I only quote the expression in its simplest form. There are more complicated similar forms, but in all cases the induction caused by the coil's own current, or the true "self-induction," is equal to something multiplied by C; and that something essentially is the coefficient of self-induction, or the inductance, a thing which is commonly denoted by L. So the above expression is simply N=LC. Hence, if we have a name for the coefficient of self-induction, we have a name at once without any trouble for the total magnetic induction through an area. I do not wish to call it any particular name, but supposing mac were the unit of self-inductance, then the unit of current being an ampere, the unit induction would be a mac-ampere ; thus we should have a name at once for the total magnetic induction, and when you want to express the intensity of a field it would be a mac-ampere per square centimetre. Another name for unit induction is a sec-volt, which is plainly equal to a secohm-ampere, or, as I have just called it, a mac-ampere. Perhaps sec-volt is a good name; of course, it is the number of lines of force which if cut in one second gives, in the cutting circuit, an E.M.F. of one volt. Mr. W. H. PREECE said the work of the British Association Committee on Electrical Standards has now extended over thirty years. Its first report was issued in the year 1861, at Manchester, and we have, after a great deal of difficulty and trouble, succeeded in im pressing those units of measurement, not only upon the scientific world of England, but upon that of all countries. During this last year, a committee appointed by the Board of Trade has taken into consideration the work of the British Association Committee and has come to the conclusion which will result, in the month of November, of an order in Council being passed making the ohm, the volt, the ampere, and the farad legal standards, such as will for years to come be regarded as the stable standard measurements in England. Now having run in a groove for 30 years, and having succeeded by dint of great labour in impressing these units on others, I am afraid we are running our heads against a brick wall when we attempt to produce any change upon such a system. Nevertheless, while the difficulty of producing a change is very great, I am going to have the temerity of suggesting a change in another direction, in addition to that suggested by Dr. Lodge. While there is a difficulty in making a change, there is also a difficulty in impressing new ideas into our language, even when absolutely essential. New wants arise and we must have new names to express these new things. The fashion to introduce new meaningless words has practically failed. They tried to thrust upon us the term barad as indicative of the standard of pressure, bole the unit of momentum, and kine the unit of velocity, but I think this attempt has practically failed, and I have met but one case where one of these names was used, and then it was spelt wrong. I am a great advocate for the use of words based on names. They enable us to pay a graceful compliment to those who have passed away. They enable us to attach a useful word to something that requires great precision of thought. The use of such words as ohm and in some cases they remove great difficulties, for I am and volt and ampere almost render definition unnecessary, fessors sometimes, it was in drawing distinction between sure if there was a mistake made by students and by pro"power" and "work." words, watt and joule-watt as the unit of power, and the unit of work; and I think I can joule as confidently state that nobody since the introduction of these words has made the mistake of confusing words clearly indicating these standards are very useful power with work or work with power. So, I say, and valuable. We have several names from which we Now we have introduced two might draw. We have Gilbert, we have Gauss, we have Weber, we have Franklin, Henry, Maxwell. These names they might be very well applied. I have mentioned the are valuable, and certainly there are some units to which names of men who are dead, for I think it would be unwise to affix the name of the living, and though there is one whose name will be associated with the electrical units some day, it is perhaps better for the present to stick to the dead. There are more names wanted still. Prof. Lodge has alluded to the unit of magnetic field or force, and there is not the slightest doubt that the time has arrived when we must adopt the unit of magnetic field, and we must give that unit a name. Some have proposed that this unit shall be based on the C.G.S. unit alone, others have suggested that it should be 10 to the power of 8, and I am going to propose that it should be called 10 to the power of 9. The reason why I suggest that change is dependent on another alteration that I am going to suggest, that the volt should be 10 to the power of 9 instead of 10 to the power of 8. That matter is dealt with in a paper by Prof. Stroud, and I would rather adhere to this point only now, because he will in his paper deal with the matter very thoroughly. Ten to the power of 8 was selected by the original committee solely because 10 to the power of 8 was the nearest approach that we could find to the Daniell cell. That cell was then universally applied, but it is now surpassed by others such as the Leclanché or bichromate, the EM.F. for which is much higher. One great reason why I propose that the value shall be 10 to the power of 9 is, that it makes the farad the same in C.G.S. as in practical units. I should very much like to have said something on dimensions, but I want to refer to one other unit that requires to have a name attached, that is H. There was a time when there was a great demand for unit magnetic pole, but that time has long since passed, and Dr. Lodge himself has referred to induction as N, more particularly in its form per square centimetre, and called B density of field. There are two great quantities that are most important in electrical developments of the present day, and that require a great deal of siftingnot that we can find units for them-one is μ, the other the coefficient of specific inductive capacity, sometimes called K, then we come to L, the coefficient of self-induction. I have said that there is more nonsense talked about selfinduction than any other branch of electrical science. There are those who talk about self-induction who have had very little practical experience of its effects, who have applied self-induction sometimes to a geometrical quantity, sometimes to permeability, and sometimes to the whole induction. It has been my practice for some years never to speak of self-induction, because it is mixed up so dreadfully, but always to speak of electromagnetic inertia, and that is the term we have met with in practice which is familiar to us. When we find writers misusing this term self-induction, it makes one regret that we have not a unit for it so as to centralise one's ideas and to avoid friction. I am at one with Dr. Lodge when he proposes a unit of self-induction. Nobody has proposed a better name than that of "henry." 10 to the power of 9 is a very good unit indeed, and if we had this unit 109 we could wipe out the use of the word secohm. The system of units now used is absolutely artificial and conventional. I will not occupy your I will not occupy your time with dealing with the relations between H and various other quantities. The name of Gauss has been proposed; in fact, it was proposed in Paris. If there are two men whose names ought to be used as units, one is Gilbert and the other Gauss, who have done so much to establish the absolute system. We want to know the value of K and R. Perhaps Prof. Rücker, who has worked in this direction, will do something more. We want something more than dimensions; we want a table of specific resistances. At the present moment we have really only the specific resistance of mercury and of copper accurately determined. We also want a new determination of J, and a better mode by which the mechanical equivalent of heat can be arrived at. We want to get rid of the obnoxious table of dimensions that every text-book is laden with. In Maxwell's great work there are three or four different systems of dimensions, and in engineers' pocketbooks, which are supposed to contain useful facts, pages after pages are filled with these useless dimensions. In conclusion, the speaker said he should be very sorry if any decision that this section should come to should interfere with the acceptance of the ampere, ohm, and farad as the legal standards. If it were to go abroad that they were disunited it might perhaps tend to the withdrawal of the order in Council. He thought that certainly this year these standards would be made legal, and then these standards would be the legal standards of the world. A paper was then read by Prof. W. Stroud on (1891) dynes is objectionable, as custom has restricted the use of Greek derivatives entirely to C.G.S. units. That 107 dynes, if required, should be called a hebdomodyne, suitably contracted of course, or preferably a joc (joule over centimetre). 2. That the classical languages are of little or no service for the provision of names for modern, more or less complex, physical conceptions; and therefore this method of coining words it is desirable to abandon. 3. That for C.G.S. units some system of automatic nomenclature, in which every name shall be self-explanatory, would prove a boon to the teacher and a blessing to the student, and that such a system is quite capable of being devised. 4. That the prefixes meizo, to indicate 109, and mei, to indicate 10-9, may be found useful. Mr. SWINBURNE said he was rather inclined to think it would be better to stick to the old units. He did not think Section A realised the enormous difficulties that they put in the way of practical people by making alterations. The alterations in the ohm would produce a great deal more trouble than was yet realised. He might say with regard to self-induction that it would not spread much in practical work. His firm made a large number of transformers, and it was necessary to make these with great accuracy, but he did not believe anyone in his place could tell in secohms what the self-induction of any transformer was. In regard to names, he would hope that the name of Poggendorf would not be left out. Dr. JOHNSTONE STONEY said on this question, at the present day, he was a complete conservative. He would wish to see the ohm system of measurement maintained intact, and he believed that only practical mischief would result from any attempts now to diverge from the ohm system. He was a member of the committee which fixed the C.G.S. system of measurement recommended in 1874, Prof. Carey Foster being another. On that committee there were some members who had long practical acquaintance with the metrical system of measurement, but there were other members who had only immediately before been converted to the opinion that the systematic system of units ought to be based on metric measures, and who had had little practical acquaintance with it as a working system. He (the speaker) dissented from the selection that was made on grounds totally different from those attributed to him in Prof. Everett's book. He would wish to take this public occasion for stating that the original note which Mr. Everett put in the first edition, and the subsequent note in the second edition, did not in the least represent what he stated at the committee. No systematic system of units could have been framed without some electrostatic and electromagnetic units belonging to it being inconveniently large or small; but it was quite unnecessary to make a selection which led to the same defect prevailing among the dynamical units. What he (the speaker) had objected to was choosing such fundamental units as led to a unit of force so inconveniently small as the dyne (about the weight of a milligram), a unit of energy so small as the erg (about the hundredththousandth part of a grammetre), and a unit of power which is the ten-millionth part of the watt. The definite proposal he made to the committee was, that the metre should be the unit of length and the kilogramme of mass; but he would have concurred in recommending any selection of fundamental metric units which would have led to convenient dynamical units. With respect to the unavoidable size and minutenomenclature should be introduced which would make it ness of some of the electrical units, he suggested that a multiples of them. For instance, they had got the farad, easy to use in practice any decimal multiples or suba unit of capacity too large for any practical purposes. Accordingly, the practice was to make use of the microfarad, as it was unfortunately called. He would prefer to call it the sixth farad (the farad divided by 106). It is a very convenient measure for measuring the capacity of submarine cables, and in cases where large capacity had to be dealt with; but if they wanted to measure the capacity 1. That the term dyne to indicate 107 of our present of an ordinary Leyden jar it would be a great deal better Present Practical System of Units. 1. The present practical system of units is very objectionable on three grounds: (a) There is no prima facie reason why the practical unit of current should be equal to th C.G.S. unit. (B) The relation between the other practical electrical units and the corresponding C.G.S. units is much more complex than need be. (y) The units of work and power are far too small for practical requirements. 2. If we were starting to devise a practical system to-day, such a system could best be formed by taking 10-9 cm. as the unit of length, 10-9 gr. as the unit of mass, and the second as unit of time. 3. That in the interests of the "practical" men of the future and in the interests of the electrical students of both the present and the future, it is highly desirable to institute a revolution with the object of dethroning the present practical system of units. Nomenclature. STONEY'S TABLE OF THE RELATIONS BETWEEN ELECTRIC UNITS. E [Voltage, or electromotive force], identical with potential on electricity. In the columns of dimensions a = √L M, B = √LM, y = LM. The units of L, M, and T for the Maxwell column are so selected that the unit of v becomes in it the velocity of light, and that the units of a, ẞ, and γ remain the same as in the ohm columns. * Asterisks are introduced to direct attention to the more important of the horizontal lines. to measure it in tenth farads instead of sixth farads. This nomenclature would enable one to use the tenth farad when convenient, and the sixth farad when convenient, and would tell exactly their relation to one another. To deal with multiples, and in order to make the distinction between sub-multiples and multiples as great as possible, he proposed that the tenth metre should mean sub-multiple, and that the metre-ten should mean multiple, and so on. That system of dealing with the multiples and sub-multiples would be a very great practical convenience to electrical engineers. It would also avoid entirely the necessity of following what, he understood, was the suggestion of the president, of using a unit for self-induction which did not belong to the ohm system. He certainly thought He certainly thought it would be a very retrograde step for the B.A. to abandon the fundamental principle of making all the measures belong to some definite systematic system. It was a misfortune that such a thing should occur as was mentioned by Mr. Swinburne, that artisans who were habitually engaged in making transformers could not tell in the least, in any definite system of measures, what the self-induction was. He believed it would be a very great increase of their capacity fordoing real work if the use of measures at every stage was so facilitated that every artisan could have a clear perception of the measures he was using. Personally he was averse to any revolu tionary change, and although the C.G.S. system he believed was not the best that could have been selected, he should be sorry to see it interfered with in any degree now. He proposed an obvious and easy method for converting from electrostatic into electromagnetic measures. There was no doubt, for some purposes, electrostatic measures were more convenient to work. At present engineers were practically precluded from doing so in a case where it would give them a great deal of assistance, by the difficulty of easily tracing the relation between the two. Mr. Preece seemed to have a great objection to dimensional equations. He (the speaker) on the other hand, would wish to see dimensional equations made so simple that they would come into very general use. The speaker here referred to a table, given herewith, which was intended to facilitate the conversion of electrostatic into electromagnetic measures. The units in the central column of this table had been introduced, not with any intention of recommending the use of a new set of measures, but only as the natural bridge between electrostatic and electromagnetic units, which assisted the conversion of these into each other by making clear to the mind the numerical relation between them. He thought that the natural name for the unit of magnetism would be the Gilbert, he being the first man they could trace who had really studied magnetism, and the natural name for the unit of magnetic field would be Gauss. Poggendorf and some others had been mentioned, though if any name deserved the distinction, it was that of Weber, who was the first person to point out the importance of systematic units. There might be a difficulty in the way of this on account of the pronunciation. He would also very earnestly try to persuade the members of both Section A and Section G to the opinion that the time had fully come for getting rid of the dreadful misuse of the word force, as in the phrases electromotive force, magnetic force, etc. The speaker, in conclusion, said he did not like the word pressure: it suggested a misleading analogy. He saw no objection to the word voltage, which was very convenient for practical use and got rid of the false analogy. Prof. CAREY FOSTER expressed his satisfaction at hearing the expression of Dr. Johnstone Stoney's conservative principles in the matter of nomenclature. If they were to have new standards they must employ a new name; they might employ a term derived from "Galvani," say "galv" or "gal." Prof. SILVANUS THOMPSON said, in relation to the unit of self-induction, that Dr. Lodge had stated that he did not see why its coefficient should be considered as a length, but it was a length, and Sir William Thomson had devised an ideal experiment illustrating the point. They needed that a physical fact should be discovered to enable them to determine the absolute value of k or p. The PRESIDENT stated that time would not allow of the continuation of the discussion; he hoped that it would be continued in the journals, especially as at present it was not quite clear what was actually wanted in practice. Mr. PREECE thought that it would be objectionable for the British Association to suggest any change in the principle of the unit. The farad was as immense as the universe, and the microfarad very small, but useful; he thought that it would be well to call the microfarad the farad, and that they should call the volt and ampere either the 10-volt and 10-ampere, or the weber. A STAPLE-DRIVER. Those who have had much experience in running bell wires have often wished for some easy method of holding staples in position without using their fingers for the purpose. We illustrate a simple little instrument, which has been designed with this end in view. It holds the staples in position until they have penetrated far enough to be driven home with the hammer. The tool consists of a rectangular tube of a size suitable for the staples being used, in which is a plunger with a movement of about in. The staple is placed in the end of the tube, when it is held, and the projecting points are placed over the wire which is being run. A blow is then New Staple-Driver. Prof. T. H. BLAKESLEY wished to introduce to the section | a unit with which he had been greatly concerned; this was the particular one which was measured when a current was sent through one coil of a dynamometer, and another current through the other. It was the product of two currents. It was most important to arrive at the power per unit of resistance. At the Physical Society he had given many cases in which the application of the split dynamometer would be the most advantageous, and in some cases, such as the power lost in the revolutions of magnetism in the iron core of a transformer, it was practically the only way to arrive at it. He was very averse to giving the names of people, however eminent they might be, to units. Prof. ANDREW GRAY thought the phrase "electromotive force was unfortunate, but that it would cause much inconvenience to drop it. A clear distinction, ought, however, always to be made between E.M.F. at a point (that was, the electric force) and the E.M.F. round a circuit. It would cause serious confusion if one word were used in two senses; if the word "force" was not used alone, but was described at the time of speaking, he did not think much disadvantage would arise. He trusted that more attention would be given to what was of greater importance than finding a name for the unit of inductance-viz., the calculation of coefficients of induction. After a few remarks from Prof. J. V. JONES, given to the opposite end of the plunger, after which the tool is removed and the staple driven home. It will be found very handy for those who are in the habit of running wires overhead, or in other places where it is inconvenient to hold the staples in the fingers. The tool may be obtained of Messrs. Woodhouse and Rawson United, Limited, 88, Queen Victoria-street, E.C. Search-Lights in Shipwrecks.-Recent disasters have called forth the suggestion that electric flash-lights on the coast should be rendered available in case of shipwrecks by night, and the special case has been mentioned that the lights under the control of the Tyne Submarine Mining Engineers should thus be used in the event of wrecks at the mouth of the Tyne. A local correspondent with reference to the magnificent flash-light at Cliffords Fort, asks: "If we use the most powerful light we have to search for our enemies, why not use the same to search for our friends who may be in peril?" He suggests that the plant at the fort might send the power both to Tynemouth and South Shields Life Brigades, where the largest and most powerful light would prove of the greatest service. At any rate he thinks it would be worth trying, as no experiments would be wasted that would in any way assist such heroic efforts as were made at Tynemouth at the wreck of the "Peggy." |