Изображения страниц
PDF
EPUB

THE

UNIVERSITY MAGAZINE.

OCTOBER, 1879.

THE NEW PHILOSOPHER'S STONE.

The three grand secrets of nature of nature. As to the third, while which it has been the great object an improved knowledge of meof the student of magic and of chanics has taught us that the proalchemy to surprise, have been the duction of matter is due, physically elixir vitæ, or universal remedy for speaking, to the transformation of all diseases, including old age; the the energy of heat, our chief adphilosopher's stone, or that agent vance of late and that a very which should convert base metal gigantic one) has been made in the into gold; and the command of construction of machines which are perpetual motion, or the means of moved by heat, such heat being applying the natural forces of liberated from the stores accumunature to the mechanical service of lated in past times by the vegetaman.

tive produce of the carboniferous In the present state of science flora of the earth, which is stored objects bearing some close relation up in the fossil form of coal. to the dreams of the alchemist form As to this method of subjecting the special pursuits of the phy. nature to the science of man, sician, the metallurgist, the ana there can be no doubt that its lytic chemist, and the engineer development hitherto has wrought In the first branch of study great the most wonderful revolution in progress has been made in the dis human history--a revolution of covery of the action of specific which we only see the commenceagents on specific functions of the ment. The practical check to human mechanism. There may be progress is the cost of the exreason for the opinion that a very traction of coal from beneath the great development will here. earth, and of the conveyance of after be given to this study. As the fuel to the place where it to transmutation, the atomic theory, is required to originate motion. to which we owe so extraordinary Beyond this lies the theoretic mis. an advance in that analysis of the chief, that the supply of coal, properties of matter which we call though enormous, is in fact limited, chemistry, is now, in its turn, on and that no method of supplying its defence ; and the idea of the stored-up heat has yet been pointed ultimate unity of matter is held by out on which the engineer can rely some of the profoundest students in the absence of coal. The saving

clause here is to be found in the pur- moving all kinds of machinery, over suit of forestry. But even the hundreds of square miles. The wisest adoption of the regulations of difficulty now opposing that utiliforestry would but ill make up for sation is solely that of transmission the want of coal.

of energy. The utilisation of the waste Our attention has recently been power of nature would be a practi- pointedly called to this question of cal solution of the problem of per- the transmission of power; and we petual motion. In the energy of cannot conceal our wonder that the tides, in the fall and rush of certain scientific evidence on that water, and in the movement of the subject which has been accessible atmosphere, are mighty forces con- for nearly two years, has hitherto tinuously in exercise, the conver- failed to receive that full attention sion of which to mechanical duty which its importance demands. would supply us with the power of The mode of transmission to millions of horses, free of every which we now refer is not by hy. charge but that of the transmission draulic or pneumatic appliances, of the power. From very early but by electrical agency. It has untimes these powers have been uti- til recently been assumed by every lised by men, in the form of tide- writer on the subject of electricity, mills, of river-mills, and of wind that a limit to the economical mills. The limits to their use have transmission of electric energy is been two: first is the irregularity imposed by a primary law of of action-the force at one time nature. The resistance of a conbeing very great, and at another ductor to the transmission of the time almost insensible; secondly, electric energy is in direct proporand more unmanageable, is the tion to its length, and in inverse difficulty of transmission of energy. proportion to its sectional area. The operations of machinery, as Thus, in order to obtain the same now at work, are required chiefly efficiency at doublea given distance, in the centres of human population. it would be necessary to double the The spots where the great genii area of the conductor as well as to of nature are ever twisting their double its length; or, in fact, to ropes of sand are for the most part increase its weight in a fourfold remote, as in the falls of mighty ratio. This fact has been expressed rivers, or on the shores of tidal seas. in the formula, that the cost of The problem of perpetual motion conductors increased as the square is not, in our time, that of setting of the distance. Such large figures the energies of nature to work at are so soon reached by this method command, but that of transmitting of progression that the subject has the force thus imprisoned to the been held to be set at rest by the spot where its exercise is required. operation of this law of resistance; If we can do that, we shall have and the idea of the economical solved the problem of perpetual transmission of electric energy for motion. For so stupendous is the long distance has, to a great extent, wasted power of nature, regarded been laid aside. from the standpoint of the engineer, In the course, however, of obserthat a very small percentage will vations made with a view to the serve all the need of man, if it can establishment of the electric light only be indeed available. One per at the South Foreland Lighthouse, cent. of the force of Niagara can Dr. Siemens has arrived at a conbe easily obtained, and would suf- clusion which, if sustained, tends fice for lighting, warming, and very materially to affect the opera

tion of this law. The proposition be necessary to lay down a second that the weight of the conductor wire or to double the cross-section must increase as the square of the of the first wire. In that case distance, is now regarded by Dr. there would, of course, be a wire Siemens as one which, though true of twice the length and twice in itself, will lead to very erroneous the area—that is to say, of four conclusions as to the practicability times the weight of the first. That of transmitting power unless fur- points, as before said, to an increase ther considerations are added. It in the weight, and approximately has been found that the economical in the cost, of the conductor in the limit to the resistance in an electric ratio of the square of the distance. machine or conductor is about one But here comes in the observaof the electric units, known by the tion that the electric conductor name of ohm; as, if that degree

does not resist the motion of elecof resistance is exceeded, a loss of

tricity in the same way as that in current ensues through the heat

which a pipe resists the flow of ing of the coil. Mr. Sabine states

water through it. An ohm's rethe conducting resistance of an sistance is an ohm's resistance. average wire of No. 4 Birmingham

whether the current flowing wire gauge (which is equal to a

through the conductor is larger or diameter of 0*238 inches) at 77 smaller. The only limit to sending ohms per statute mile. The re

more powerful currents is the heatsistance on the electric telegraph

ing which they cause in the concurrent from London to Birming

ductor. According to Mr. Sabine, ham is stated by Mr. Preece as

this resistance increases 0.2 per 1500 ohms (written 1500w). The

cent, for each Fahrenheit degree of dynamic equivalent of the ohm is

temperature in the conductor. As not mentioned by Mr. Sabine in

the fourfold conductor would have the work referred to, or by Mr.

four times the superficies of the Preece in a larger volume. The

original conductor, the dissipation volt, which is the unit of electro

of heat would be also four times motive force, as the ohm is of re

the amount of that occurring in the sistance, absorbs 44:24 foot-pounds

first instance; and thus, says Dr. per second. 1 H.P. is equal to 766

Siemens, the doubled conductor volt-ohms, and is equivalent to the

would be capable of transmitting consumption of 74.86 grammes of

four times the amount of electric zinc per second in a Daniell's cell,

current. or 895.2 grammes per hour. If, therefore, argues Dr. Siemens,

Dr. Siemens adds that, if one there is a machine with one ohm

wire of double area be substituted resistance, there ought to be a con

for two wires, the superficies will ductor transmitting the power with

not be doubled, but will only be a resistance of not more than one increased in the ratio of 2 /7 to l; ohm. If, instead of conveying the and the relative transmitting energy for one mile, it is desired to power will not be increased in a carry it for two miles, the length higher ratio than that of 1 to 2vo, of the conductor will be doubled. or 1 to 2.83. If the obstruction But by this change the resistance arises from the actual temperature will also be doubled, or become of the conductor, it would be reequal to two ohms, and much of duced by substituting thin laminæ the useful effort of the original of metal for a solid wire. If it be force will thus be lost. To reduce actual conversion of heat, it is the resistance to one ohm it would difficult to see how the more or less free radiation and escape of the One-tenth of the total power em. heat can reduce the loss of power ployed would, therefore, go to heat in the current.

the conductor; and if that conThis view of Dr. Siemens, when ductor is exposed to the cooling brought forward by him at the In. influence of the atmosphere, it stitution of Civil Engineers, in would be able to transmit a vast January, 1878, was at once hailed amount of electric energy. Dr. by Sir William Thomson, the Pro. Siemens concluded by stating it as fessor of Natural History at the his opinion that a conductor of two University, Edinburgh, who was inches diameter would probably for five years President of the suffice to convey electric energy Royal Society of Edinburgh, as equal to 1000 horse-power to a “quite new, and of great practical distance of thirty miles. The importance.” The question of the general testimony is, in his opinion, heat developed in the wire was, Sir that about fifty per cent. of the William remarked, the fundamen- motive power is lost in transtal question with reference to the mission. quantity of metal required to communicate the effect to a distance.

On the 23rd May, in giving It appeared to him that the most

evidence before the Select Compractical way of producing the

mittee on electric lighting, Sir W.

Thomson reiterated the views he result would be to put the wire in

had expressed at the Institution of the shape of a copper tube. A

Civil Engineers, entering further current of water might be made to flow through this tube, and to

into detail. He told the com. carry off the heat as it was deve

mittee, “I look forward to the

falls of Niagara being extensively oped. In April last, in giving evidence

used for the production of light before the Select Committee on

and mechanical power over a large lighting by electricity, Dr. Siemens

area of North America." He conagain stated that the only limit to

sidered that this power might be the transmitting power of a long

“advantageously transmitted for conductor is its liability to become

hundreds of miles.” He “had heated; for, “in transmitting an

no limit to the application of electrical current through a con.

it on a great scale." It could do ductor, a portion of the dynamical

all the work which is done by steam effect of the energy is lost and

engines. Sir William furnished a converted into heat, which heat

carefully written reply to several accumulates in the conductor, and

questions as to the size of the conhas to be disposed of by radiation

ductors, in which he gives a foror conductor.” Here direct loss

mula taking the resistance in ohms is mentioned; nor is there reference

from Clark and Sabine's electrical made to any electrical resistance

table and formulæ as indicated by caused by a heated conductor, the expression ; when I is though physical inconvenience the length of the conductor in staseems to be anticipated. If the tute miles, and d is its diameter in resistance of the conductor is made tenths of an inch. Sir William has equal to that of the dynamo-electric come to the conclusion, “ assuming machine itself, it follows, Dr. 20 per cent. to be lost in transit," Siemens says, from the recent that a copper wire of half an inch experiments of Dr. Hopkinson, in diameter would transmit 21,000 that the loss does not exceed ten horse-power for a distance of 300 per cent. of the power employed. statute miles; the power sent in

being that of 26,250 horses, and energy. We have done so with the that brought out being 80 per cent. more anxiety from the fact that of the same. But no explanation contradictory and unintelligible is given as to why 20 per cent. loss statements have lately appeared on is “ assumed." Dr. Siemens, as we the subject. But whatever be the have seen, gives 50 per cent. as the cause, there is far more obscurity proportionate efficiency, which might than we could wish in the state. be further reduced by the develop. ments of the eminent men who so ment of heat. Sir William Thom- recently gave the evidence which son's formula can only give useful we have cited. One reason, no results when it is shown what are doubt, may be, that the inquiry of the facts on which the value “ lost the committee was recently directed in transit" is calculated. Sir to lighting by electricity, while our William calculates (on the assumed own inquiries have been into the 20 per cent. loss) that the genera- far broader and more comprehention of heat in the main conductor sive question of the transmission of will be at the rate of about do of power. But it must be remembered, a French thermal unit per square on the other hand, that all the centimètre of surface per second. numerous and complicated quesThe rate of loss of heat by variation tions as to regulators, lamps, and and convection he gives as about generative machines, are laid on 1000 per square centimètre per de one side in our investigation. What gree of excess of temperature above we seek to do is to find how far, that of the surrounding medium. from accessible sources of informaThen, he argues, the main con. tion, the idea of the absence of ductor, if freely exposed to air, will direct functional relation between be raised only 20 degrees Centigrade the quantity of energy transmissible by the heating effect of the cur. and the resistance to transmission rent, and the use of tubular con is established. The economical ductors will be unnecessary. In properties of the resistance of this communication Sir William circuit and machine is laid down. gives the expression, P= the pro The effect of length and area of portion of work lost in transit, in conductor on resistance is previrtue of the resistance of the con- scribed. What we want to underductor. But as all that he says as stand is, with a given conductor, to this is “suppose here 20 per what will be the increased resiscent. to be lost in transit,” we do tance of an increased current. not obtain anything more from the As to this, it is disappointing to working out of an equation con. find that, after announcing their taining expressions raised to the accord, eighteen months ago, Dr. 24th power, than is involved in Siemens and Sir William Thomson that supposition. It is very tan- are now so wide in their reckon. talising to be presented with so ings. The one requires for transevidently imperfect a paper by the mitting an energy of 1000-horse printer of the report of the com- power for the distance of thirty mittee.

miles, a conductor of two inches We have gone with considerable diameter, with a resulting loss of care into the exact language em. 50 per cent. of initial energy. The ployed by the most eminent men other gives, for transmitting an who have recently expressed their initial energy of 26,250 - horse views on this highly-important power for a distance of 300 miles, question of the limitation of the à conductor of half an inch transmissibility of the electric diameter, and a loss in transit of

« ПредыдущаяПродолжить »