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are to be made of steel or of different these causes, experiments should be made brands of plate iron of known quality; with flues constructed in the same manothers are to be made with welded joints ner and of the same material as the flues instead of riveted butt-joints at the lon- of the first series of experiments. The gitudinal seams; in others, different ratio of thickness to diameter should be kinds of circumferential joints connect- likewise in them approximately the same, ing the several sections of the flues may but their cross section should deviate a be used. At least two experiments should known amount from the circular form. be made with each kind of material and In determing this deviation it is not sufstyle of joint, but the dimensions of the ficient to measure the differences of difiues may be varied in each experiment. ameters, but the actual curves of the
A third series of experiments should cross sections should be accurately measbe especially directed to the investiga- ured. To make the results of these extion of the weakening effect of devia- periments comparable, at least two tions from the circular form. This sub- values of sufficient magnitude should be ject should receive the most careful at- given to these deviations from the circutention, as an accurate determination of lar form in otherwise identical flues. this weakening influence will enable us A fourth series may comprise experito proceed intelligently in assigning a ments with flues made of old plates corco-efficient of safety, to be used in con- roded to a greater or less degree in an nection with formulæ derived from the irregular manner, or showing signs of first and second series of experiments, blistering and lamination. In others and may likewise explain any apparent local weaknesses and unequal distribution anomalies in the results of these experi- of strains may be produced by the apments. Slight deviations from the cir- plication of intense heat. For the latcular form are unavoidable in furnace ter purpose gas jets, impinging on the flues, and will occur also in the experi- crown of the flue may be used, the exmental flues of the first and second ternal cylinder having been filled only series; they must be carefully measured partly with water so that the space over and recorded in all cases. În practice, the top of the flue is filled with comdeviation from the circular form may be pressed air, which permits the overheatdue to the following causes, viz.: 1. ing of the plates. The mode of construction, e. g., when From every plate used in the construclap-joints are used; in this case the tion of the experimental flues two speciamount will depend on the thickness of mens should be cut, one lengthwise and the plates. 2. Inaccuracy of workman- one crosswise the fiber, for testing the ship frequently causes a difference of 1 ductility and the limit of elasticity of inch or inch" in the diameters of flues the metal in the testing machine. These of ordinary dimensions. 3. Distortion specimens should have a uniform width, under pressure in consequence of want and a length of 8 inches between the of homogeneity of the material, or of shoulders, this being the standard length unequal distribution of strains. No of test-specimens adopted by the French numerical value can be assigned to dis- and English Governments. The elongatortion due to these causes. 4. Distor. tion of the specimens under increasing tion due to unequal expansion in conse- strains, after the limit of elasticity has quence of difference of temperatures. been passed, should likewise be carefully The increase of diameter, due to a uni- measured and recorded. If possible, form increase of temperature, such as these tests should be repeated at high may occur in practice, is small.
The di- temperatures. ameter of a 36-inch tube will increase The thickness of each plate must be about } inch, when the temperature is accurately gauged and its soundness raised 550 degrees Fahrenheit. Local must be carefully tested before it is used distortions, due to overheating of plates in the construction of flues. If the flues in places where the accumulation of fail by local distortion, the bulged plates scale
prevents the ready transmission of should be drilled or cut and carefully exheat to the water, will vary greatly in amined for hidden defects. amount under different conditions.
The ends of the flues should be secureTo determine the weakening effect of ly fastened, as in actual boiler practice.
The apparatus used in the experiments full effect. A complete record of all made at the Washington Navy Yard, in variations in the form of the tubes un1874, may serve as a model for futuer der pressure, and of all conditions and experiments. (See Section 2 and Shock incidents of the experiments is to be “Steam Boilers," p. 113).
kept. The pressure should be applied gradu The form of the tube should be deterally to the experimental apparatus by mined before commencing the experimeans of a force pump. After each in- ments as well as during its progress, by crease of 50 pounds, or 25 pounds as the measurement from a fixed central axis limit of strength is approached, the tube by means of an adjustable radial arm, is to be accurately gauged at several movable along and around this axis. places; then the pressure is to be re While complete series of experiments, leased, and the flue is to be gauged as above described, can be undertaken again to determine whether any perma- only under the auspices of the Governnent set has taken place. The element ment or other public institution, each of time deserves particular notice in single experiment conducted by private making these experiments. Especial parties will be of great value if the care is to be taken in every case to main- above suggestions are followed, and a tain each increase of pressure a suffi- full record of accurate observations is ciently long time to let it produce its kept and made public.
ELECTRIC LIGHTING: THE DIVISION AND REGULATION
OF THE ELECTRIC CURRENT. *
From Sawyer's “Electric Lighting by Incandescence."
To announce a new work on electric X., the latter being devoted to regulalighting in these days is to excite anew tors and switches: the public interest in a subject which at no time within the last two years has
“DIVISION OF CURRENT AND LIGHT.-Much failed to elicit the absorbed attention of has been written concerning the loss of all classes. The opinion of an expert in light by subdivision of the current, and matters relating to the present and this has been variously estimated, someprobable future condition of this
times as the square and sometimes as practical question will, in some measure, the cube of the number of lights among satisfy the demand for better knowledge which the current is divided. Upon of the problems which the inventors have what data and with what purpose these of late been striving to solve.
estimates have been made it is difficult The new work of Mr. Sawyer will be to conceive, for they have no foundation found valuable, inasmuch as it describes in practical fact. with exceptional clearness the character
sf to the conducting pipe of a gas sysof the difficulties to be overcome, and tem a given volume of illuminating gas the methods devised by different invent- is supplied in a given time, and all this ors to meet the requirements of in-door gas is consumed in a single burner in lighting by electricity.
order to yield a given light, when we After describing the different types divide the volume of gas thus supplied of generators and of incandescent lamps, among two or more burners the total the author proceeds to the elucidation of light produced may, indeed, have greatly the all-important question of the “ Di- decreased. We do not, however, supply vision of the Current and Light.” gas in this manner, but the volume of We quote here from chapters IX. and gas supplied is in direct proportion to
the number of burners to be fed. * Electric Lighting by Incandescence and its applica What is true of gas is equally true of tion to Interior Illumination; dy William Edward Sawyer. New York: D. Van Nostrand.
electricity. If a fixed volume of current,
sufficient for one light, is furnished to a rent generated is divided between the single lamp the maximum effect is pro- transmitter and the receiving instrument, duced, and if we divide this fixed volume and in some large telegraphic stations the among two or more lamps the total effect wires radiate from a single generator in is greatly diminished; but to suppose several directions, and in the circuit of that such a division is contemplated is each of these wires, at towns and cities to suppose a similar operation in the case along their route, there is placed a greater of gas, and criticism of statements based or less number of instruments, each of upon such a supposition is, simply, waste which is energized from the source comof time and labor. When we increase mon to all of them; and the proportion the number of lights in circuit, we in- of current supplied to each instrument is crease the volume of current in propor- made substantially the same in all cases, tion; and the power of the total light is by making the resistance of the instruincreased in proportion; and the energy ments uniform. The strength of the expended in producing the light, and current thus supplied is, of course, inadtherefore its cost, is increased in propor- equate to the operation of an electriction.
lighting system, as an ordinary main-line Without at this time entering into any telegraphic battery is inadequate to the considerations of the fact that the cur- operation of even a single voltaic-arc rent from a single generator of electricity lamp ; but if we increase its strength has repeatedly been divided between proportionately, and for each telegraphic from two to two hundred incandescent instrument substitute an electric lamp, burners, the operations of the Brush sys- we as certainly accomplish subdivision tem of voltaic-arc lighting set at rest the of the electric-light current as subdivision question of economical subdivision. of less powerful currents is accomplished
Two machines of the Brush type are with telegraphic instruments. Indeed, selected for comparison, viz., the six- the laws which govern the supply of gas light and the sixteen-light machines, both to gas burners and the supply of electriof which are in practical use throughout city to electric burners are daily recogthe United States—the light of each of nized and availed of in the operation of the twenty-two lamps being of the same telegraphic circuits. intensity as that of each and every other For instance, in operating a given lamp.
number of telegraphic instruments in With the six-light machine the total series, we employ a given number of cells driving power absorbed per minute is of battery. Suppose, now, that we double 236,940 foot-pounds, or 39,490 foot the number of instruments and the repounds per lamp.
sistance of the circuit; all the instruWith the sixteen-light machine the ments, since they receive proportionately total driving power absorbed is 618,090 less current, become practically inoperafoot-pounds, or 38,630 foot-pounds per tive, and we increase the number of cells, lamp.
and therefore the electro-motive force of It is thus clearly shown that for each the battery, in proportion to the addilamp added to the circuit there is an ex- tional work required, which is that of penditure of power in proportion to the overcoming the added resistance. If, additional work, the somewhat dimin. on the other hand, we divide the current ished power per lamp expended in the from a single generator among two or sixteen-light machine being mainly due more lines, each including a number of to the element of friction, in which the instruments in series, we require a curpercentage of absorption of power is rent of greater quantity; hence we inless in large than in small generators. crease the size of the elements of the
As a matter of fact, there is no limit generator. to the divisibility of the electric current. In electric lighting there are five While the possibility and the impossi- methods of dividing the current from a bility of its divisibility become from time single generator of electricity : to time the subject of controversial dis 1. The series system. Passing the cussion, practical subdivision is a daily current seriatim through the lamps, as concomitant of every telegraphic circuit. in the Brush system of lighting. Even in telephonic transmission the cur 2. The multiple system. Connecting
the poles of the generator with two par- other forces, direct application is found allel wires, and placing each lamp in a to be the most economical. branch running from wire to wire, as in How to practically divide the current the Edison and Maxim systems.
from a single source among a large num3. The multiple-series system. Con- ber of incandescent lamps has been connecting the poles of the generator with sidered a debatable question. We shall two parallel wires, and placing a number content ourselves with glancing at the of lamps in each branch running from results which must follow an extension wire to wire, as in the Sawyer-Man sys- of the four systems which appear to be tem.
suitable, assuming that in each case 1,000
14. The series-multiple system. Con- , lamps and 10,000 lamps are to be operated necting one pole of the generator to a upon a single circuit. Where there is a wire which, at the point at which light limited number of lamps either the series, is needed, divides into a number of the multiple, the multiple-series or the strands, each containing a lamp or lamps, series-multiple systems may be employed and which strands again combine to with about equal advantage; but where gether in a single wire, which runs to the number of lamps is increased to hunthe next point of division ; and so on in- dreds or thousands, as must be the case definitely, returning finally to the other in any general application of electric pole of the generator ; as in the Sawyer lighting, considerations novel in characsystem.
ter rapidly present themselves.
Fig. 67. Lamps in Series.
5. The secondary system. Passing: Taking first the series system (Fig. the main current through the primary 67), in which the current traverses the wire of an induction coil, in the circuit lamps seriatim, and assuming that interof the secondary coil of which the lamp ruption of the circuit of any lamp will is placed (Fig. 66).
not interrupt the entire circuit, we have Alternating or intermittent currents with 1,000 lamps a resistance, not includare employed, and, owing to the reactive earth inductions, this principle cannot
* As recently as 1877-78 several claimants to this be applied over any considerable terri- method of subdivision have appeared, but the system tory. Moreover, it involves loss of power Patent 685), under the title," Improvements in obtain. in the heating of the iron core of the in ing Light by Electricity;" and it is not quite clear that duction apparatus, is an indirect applica- patent expired in 1971, unless the Government taxes tion of power, and, in electric as in all were unpaid, in which case it must have expired at
an earlier date.
ing the internal resistance of the gen- ployed), the internal resistance of the erator or the external resistance of the generator would be 66.66 ohms, and the main conductor, of 1,000 ohms. With total resistance of the circuit 266.66 10,000 lamps the resistance becomes ohms. Suppose now that we add another 10,000 ohms.
lamp to the circuit of the machine; we Can this resistance be overcome by reduce the external resistance to 100 any practicable construction of gen- ohms, because the current has now two erator?
paths to traverse, each of 200 ohms reThe electro-motive force of current sistance. The total resistance of the necessary to operate an incandescent external circuit becomes 166.66 ohms, lamp of one ohm resistance is, as to the 66.66
and lamp, such as will yield a voltaic arc
of the current is wasted in
166.66 of an inch in length. The electro-motive the machine. Let the number of lamps force necessary to overcome the resist
be ten; then we have an external resistance of 1,000 lamps is, therefore, that which will yield an arc of 317 inches in ance of 20 ohms, and a total resistance length, or with 10,000 lamps an arc 26 of 86.66 ohms. We thus obtain in the
20 feet in length.
external circuit only of the current Assuming its existence, we need not
86.66 describe the probable effect of a current generated, or about 23 per cent. Exof such tension upon the person of any tending the calculation to one hundred one unfortunate enough to come in con- lamps, thus making the external resisttact with the conductor, or the difficulty ance two ohms, we are able to utilize less of insulating the conductor at all; but than 3 per cent. of the current generated.
Fig. 68. Multiple Circuit.
will simply say that no generator could In order to be able to operate one hun be constructed around the commutator dred lamps with a utilization of 50 per of which the current generated would cent. of the whole current, we must, not short-circuit. In other words, the therefore, reduce the internal resistance current could not be produced exterior of the generator to two ohms; with one to the machine.
thousand lamps, its resistance must be With the multiple system (Fig. 68) the reduced to two-tenths of one ohm ; and conditions are in some respects reversed. with ten thousand lamps, to two oneThe danger of a short circuit occurring hundredths of an ohm. In order that in any branch is avoidable, and the elec- there may be but little loss in the main tro-motive force of current being low, conductors leading from the generator, there would appear to be no difficulty in they must be of large size, for the resistinsulating the main conductors. The ance of a copper conductor weighing four questions arising in the operation of pounds per foot is substantially four this system relate particularly to the one-hundredths of an ohm per mile; and generator.
as there are two mains, the total resistAssuming that the generator is so con- anc) of the conductors, costing, as bare structed as to give a useful effect in the copper at 30 cents per pound, $12,672, external circuit of any percentage of the is eight one-hundredths of an ohm. With total current, say 75 per cent., and that such a conductor, the total resistance of the resistance of the lamp used is 200 the circuit of 1,000 lamps would be .48 ohms instead of one ohm (for compari- of an ohm, divided as follows: Genson with a low-resistance lamp would erator, .2; mains, .08; lamps, .2. There obviously be unfair to a system in which would thus be wasted in the generator lamps of high resistance only are em- 41} per cent. of the current, and in the