The Engineer. 379

Nov. 25, 1881. THE ENGINEER. 379 THE PARIS ELECTRICAL EXHIBITION. electro-motor, or machine for converting electric energy of the other two instruments when only the horse-power into mechanical work, we have to measure the current in expended in an electric circuit is required. This instru No. XV. Amperes* passing through it, and the electro-motive force ment —which, like the two previous ones, differs from all The great and increasing interest taken in all that con in Volts maintained between its two terminals. The pro other instruments previously designed, in that the calibra cerns the electric light, and the facilities offered by the ducts of these two multiplied by 0-00134 gives the horse tion can be effected with much smaller powers than it is Paris Exhibition, induced us to supplement our ordinary power expended in the machine. Part of this will be required to measure—will be found shortly described in a lecture delivered on the 24th March of this year, at the descriptive report of the exhibits by a special report wasted in heating the wires in the motor, part in over coming mechanical friction, and the remainder, which may Society of Arts, as well as a dispersion photometer, arranged on the electric light. Professors Ayrton and Perry be measured by a force dynamometer, can be employed in by us to measure even strong electric lights in a small having designed special instruments for making measure doing useful work. space, and which has been employed by us, conjointly with ments with these intense lights, and having given great Such experiments —which, it must be remembered, are other photometers, in the experiments contained in this attention to the subject, were requested to prepare this wholly independent of the goodness or badness of the mode report. special report. It will be found below. It is by no of generating the electric current, or of the amount of heat produced in the wires connecting the electro-motor with the means exhaustive, nor is it intended to be. It deals for generator —have been made by our students during the the first time with questions that have hitherto been care last twelve months on a magneto-electric Gramme machine, off fully avoided, but which are of great importance. The running at different speeds, with various loads. V general public necessarily look mainly to the question of In the same way, if it be desired to measure the efficiency •r — of an electric lamp, it is merely necessary to measure the PSA's total cost. How much candle-power do we obtain for a i current in Amperes passing through it, and the difference given lump sum ? The answer to this question might ) of potentials in Volts between the terminals of the lamp. wm easily be given, but, as a rule, a simple reply at present The product of these two, as before, multiplied by 0'00134, L would be inconvenient. The electric light engineer ’s gives the horse-power actually expended in the lamp, quite rr...’- =§= position is altogether different. He wants by experiments independently of the horse-power actually employed in . and expei ’ience to find out the weak places in a new generating the current. With the horse-power expended system, and to remedy its defects. Hitherto very elaborate in the lamp must be compared the simultaneous illuminat experiments have been made and published in most cases only ing power of the lamp in standard candles, and thus we DISPERSION PHOTOMETER. as regards the steam motor and the dynamo machine ; but arrive at the number of candles per horse-power expended now Professors Ayrton and Perry give us figures relating in the lamp itself. The special form of our dispersion photometer, which to the lamps as lamps. It will, no doubt, be carefully Such experiments, also, our students have been engaged we employed at Paris, is shown in the accompanying noticed that the authors seem to look upon some of their at during the last twelvemonths with a Duboscq-Foucault engraving. It consists of a wooden box, A B, to which is ineasurements as tentative, and hence not to be taken as and with a Serrin lamp, with different strengths of current, joined a wooden tube B C, the joint being covered over expressing the absolute value of a lamp. They say, “ when and with different fixed distances between the carbons ; with soft black cloth, to prevent extraneous light entering measuring the strength of the lights at Paris we were not as well as, during the spring of this year, with incandescent the photometer ; A B contains the standard candle, for able to extend our experiments on each lamp over a lamps ; and a sample of the results is given in Table I. which I) is the chimney. This candle illuminates a screen sufficient length of time to obtain an average result, In making such experiments it is of great importance to of tracing paper, while an exactly similar screen, cut from independent of fluctuations of the light, due to the adjust be able to measure both current and electro-motive force the same sheet of paper, is illuminated by a beam of the ment of the lamps not being the best for the particular rapidly with accuracy, so that all sudden changes may be electric light passing down the wooden tube B C, and dis strength of current passing through it.” Whilst then we recorded. For this purpose we have devised a dead-beat persed in passing through the thin double concave lens at H. look upon the figures in Table II., where only one series is galvanometer, which measures the strength of the current The two pieces of tracing paper are, by means of mirrors, given, as altogether preliminary, we think it cannot for a directly in Amperes, but which can be calibrated by the placed at exactly the same angle, reflected so as each to moment be disputed that the figures in the remaining use of a single Daniell ’s cell. This instrument —which we cover half the opening B, care being taken to avoid tables, however they quay be considered, do forcibly and have called an Ampere-meter, or, for simplicity, an “ Am parallax by placing the head so that the intersection of the conclusively show — meter”—will be found described in the “Journal of the two mirrors is exactly in the centre of the opening. The (1) That the ordinary ideas on photometry need to be Society of Telegraph Engineers, ” No. 36, vol. x., 1881. wooden tube, B C, can, by means of the jointed rod E F, be carefully reconsidered. If these measurements show any The proportionality of the deflections of our Am-meter placed at any angle with the horizon, so as to receive a beam thing, they show the simple photometric apparatus of with current may be easily tested, without the necessity of from the electric light at any specified angle. Rumford, &c., to give inconsistent results. We can quite using strong currents, by turning the commutator to series, When using the instrument the tube B C is pointed at the understand the results obtained by one method not agree since of course the relative values of the deflections must electric light, so that the axis of the tube appears to pass ing with those obtained by a second method ; but the results be the same for the instrument in series as in parallel through the arc itself. The concave dispersing lens is now of each method ought to be consistent. The whole circuit. As a check to this, however, we thought it well moved parallel to itself along the tube by turning the question of photometry is one of great difficulty, and of at Paris to place three of our Am-meters in circuit with a handle H, until the illumination of the two screens is equal, great importance, and the Paris Congress would have been the hole at B being closed far better employed if its members had spent their whole by either the red or the time in discussing this matter, and agreeing upon a simple green glass respectively. practical system, than in changing the name of the Weber The intensity of the light in 3 OB to the Ampere, and debating about still more abstract 30 candles is then read off questions. directly from the position (2) That the economy of any incandescent system OBS on the scale of the pointer increases with the temperature to which the filament is moving with H. It will DECREES raised is conclusively shown, the figures relating to the be observed that there are IN Maxim lamp being very exhaustive, which is the more y three such scales passed satisfactory as hitherto little has been known concerning A ■ over by the same pointer, this lamp. go the bottom one is used jy when the light is compara THE EFFICIENCY OF ELECTRIC LAMPS. AM -M ETERS tively weak, and when it By Professors Ayrton and Perry . is lift, from the end of BEAT the photometer, the middle In accordance with your request, we send you a report on the efficiency of a few of the more important systems 'Q scale when the light is M of electric lighting exhibited in the Paris Exhibition. DEAD stronger and requires that Any system of electric lighting must consist of three parts 10 the end of the photometer OUR shall be 3ft. away from —the engine producing the mechanical power, the electric machine for converting the power into electric energy, ON it, and lastly, the top scale and thirdly, the lamp for converting the electric energy for the strongest lights into light.

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