« EelmineJätka »
force in volts between D and F, i. e. the ' - E of the formula above; s denotes the radius in centimetres of the fine wire; the other lettors have the same meanings as before):
The formula by which the limit to h is calculated is
whero Ax is the difference between + + and + – balances (sce above). In the four experiments discussed, the arrangement was abundantly sufficient to indicato a difference of a millimetre, so that Ax is
< 106 ohm ;
10‘yR(PE) Assuming, then, that tho heating, and cooling-effect discussed above may be neglected, the result of the experiments is that h is certainly less than 10 In other words, if we have a conductor whose section is a squaro centimetre, and whose resistance for infinitely small currents is an ohm, its resistance (provided the temperature is kept the same) is not diminished by so much as the nia part when a current of a farad per second passes through it.
With regard to the heating, and cooling-effect, it must evidently be very small, since it takes place, if at all, in something like the lo part of a second. It is of course possible that theso alternations were at that particular rate for which the two effects would balance each other ; but when we consider that the temperatures of the thin wire were very different in the different experiments (notably so in (3) and (4) with the iron wire, where the current passing was in one case more than double that in the other), and that the heating, and cooling-effect must depend on the temperature of the wire, while the other is independent of that as well as of the rate of alternation, tho probability that any such balancing of the two effects existed at all is reduced to almost nothing. We may therefore look on this experiment as a verification of Olim's law to the degree of accuracy indicated above.
APPENDIX. While thinking how to repeat Dr. Schuster's experiments as nearly as was possible without the command of a sine-inductor, the writer of the Report
was led to try a third experiment in verification of Ohm's law. If there be a periodic variation of the primary of an induction-coil, the time integral of the electromotive force in the secondary through one complete oscillation will be zero; but if the variation consist of a sharp break, although this law holds, yet the oscillation in the secondary may be divided into two parts, in one of which the maximum intensity is very much greater than in the other. If it be true, then, that a more intense current encounters less resistance than a less intense current, clearly the law above stated can no longer hold; the law has, in fact, been deduced on the supposition that the resistance is independent of the strength of the current.
It follows, therefore, that if we send the induction-currents from the secondary of an induction-coil, whose primary is made and broken by a tuningfork, through a helix of fine wire to make sure of bringing out the effects we are looking for, then the needle of a galvanometer introduced into the secondary will be deflected so as to indicate a current in the direction of the current due to breaking the primary.
Certain anomalous, and at first sight contradictory, results led the writer to study the behaviour of a galvanometer under these circumstances. The result was the suggestion of a theory which explained the anomalies completely, and indicated the existence of certain other phenomena which were afterwards observed.
The results are, so far as the writer has been able to learn, partly new, Although not of sufficient importance in connexion with the present subject to require detailed mention here, yet it was thought best to state the results so far as they bear on the question, reserving a detailed account for publication elsewhere *.
It was found that, under the circumstances indicated above, the indication of a galvanometer is a function of the ratio of the strengths of the magnetic field when there is no current and when the currents are passing, and also of the position of equilibrium of the needle when there is no current.
Theory and observation give alike, among others, the following peculiarities :
1. If the ratio of the magnetic forces due to the currents to that acting on the needle when there is no current does not exceed a certain quantity, then if the position of rest of axis of the needle is inclined at an angle a(<90°) to the plane of the coil-windings, the effect of the alternating currents is to increase that angle, so that, according as the needle is deflected one way or the other by means of the deflecting magnet, we get opposite effects.
The effect is zero when a is zero.
2. If the above-mentioned ratio exceeds a certain value, the position of the needle parallel to the windings (i. e. for a=0) becomes unstable, and there now appear two positions of equilibrium of equal inclination either way to the coil-windings. Either of these the needle will take up and keep if brought there with sufficiently small velocity.
The greater the ratio, the more nearly these two positions approach to parallelism with the plane of the coil-windings.
The last-mentioned phenomenon was described long ago by Poggendorff, under the name of " doppelsinnige Ablenkung," and was and has been regarded apparently as an unstable phenomenon.
The first-mentioned form of the phenomenon has not, so far as the writer knows, been hitherto described anywhere. In repeating Dr. Schuster's experiments by superposing a small current
* Phil. Mag. [v.] vol. ii. p. 401.
of constant direction on the alternating current, the writer has never been able to detect any effect that could not be explained by the above results. He has not been able to use a sine-inductor as yet, so that a completo discussion of Dr. Schuster's results from this point of view has not been possible.
The strong analogy of the phenomena to those obtained by Dr. Schuster, and the fact that it has been found possible to produce the phenomenon in three different galvanometers (it is of importance to remark that the needle was elongated in all cases where the effect was strong), must, however, be regarded as affecting the probability of conclusions drawn from experiments of this kind about the truth of Ohm's law.
Report of the Committee, consisting of the Rev. H. F. BARNES, H. E.
DRESSER (Secretary), T. HARLAND, J. E. HARTING, T. J. MONK, Professor NEWTON, and the Rev. Canon TRISTRAM, appointed for the purpose of inquiring into the possibility of establishing a “ Close Time” for the protection of indigenous animals, and for watching
Bills introduced into Parliament affecting this subject. Your Committee has the pleasure of stating that Mr. Chaplin, M.P. for Mid Lincolnshire, lost no time in fulfilling his promise, announced in its last Report, and immediately on the meeting of Parliament introduced into the House of Commons the Bill for the Preservation of Wild Fowl, which had been prepared by your Committee, and has been referred to in its former Reports.
In order to aid Mr. Chaplin's efforts and to explain the objects of the Bill, your Committee in February last issued and extensively circulated the following statement:
« The Committee deems it expedient to offer a summary of its former Reports, and a statement of its present views, in regard to the probability of action being taken in Parliament during the ensuing Session for the attainment of further protection of birds.
“It has long since been stated by the Committee and the statement is beyond contradiction—that the birds which are comprehended under the common designation of Wild Fowl have, of all others, with the exception of Birds-of-prey, most rapidly diminished in numbers throughout the United Kingdom, and it cannot be doubted that their decrease is still going on.
“ The reasons which hinder the Committee from recommending any legislative protection to Birds-of-prey are almost too obvious to need explanation. The Committee, while believing the existence of such birds in certain districts, and in numbers which are not excessive, is beneficial, is aware that the contrary opinion is very strongly upheld by a large class of persons, and is fully persuaded that were it possible to pass an Act for the protection of these birds, its enforcement in a single instance would give the signal for an agitation for its repeal, which would seriously damage the cause of birdprotection in general.
“On the other hand, no charge of injuriousness has ever been brought or, if brought, could possibly be maintained—against Wild Fowl as a whole ; while the employment that their capture affords to a considerable portion of the population, and their utility as an article of food to almost the whole community, render their protection highly desirable from an economical point of view. The notorious and rapid decrease in their numbers is to be ascribed to causes that may be classed under two heads :-(1) •Indirect' and (2) • Direct.'
"(1) The indirect causes of the decrease of Wild Fowl are attributable to the diminution of their breeding-haunts by draining, the reclamation of waste lands, and agricultural improvements generally; and with these it would of course not only be impossible, but manifestly improper, for tho legislature to interfere, for with them the prosperity of the country at large is intimately bound up. So far, then, as regards their effects, the birds must take their chance.
“(2) The direct causes, on the other hand, are as plainly capable of control, for they are attributable to the destruction of the breeding-stock, and chiefly by the gun. As soon as birds pair in the spring they lay aside much of their habitual caution, and become easy victims to the gunner. Long after the pairing-season has begun our markets are plentifully stocked with Wild Fowl of every description; and it is obvious that every pair of birds killed at that time of year signifies the destruction of a whole brood, as well as that of its intending parents.
“Wild-Fowl shooting gives, as has been above stated, employment to a large number of men, who make a profession of it. These men, however, aro accustomed to certain restraints in pursuing their vocation. They are all compelled to take out a gun-license, and many of them are aware that they are prohibited from exercising their calling in certain waters and over certain lands. The notion of restraint to them is, therefore, not new; and the Committee believes that the most intelligent of them would gladly recognize the propriety of a well-considered and stringent measure, that by effectually protecting Wild Fowl during the breeding season would secure to them a greater abundance at other times of the year.
“ The Wild Fowl, for whose protection a more stringent measure is now about to be proposed, are, it is true, already named in the • Wild-Birds Protection Act;' but owing to their marketable value being greatly in excess of the penalties which that Act prescribes—very properly, may be, in regard to the other birds it names—they enjoy little or no real protection therefrom.
“ The great success which has attended the working of the Sea-Birds Preservation Act,' in which the penalties are much higher than in the • WildBirds Protection Act,' encourages the Committee to believe that an Act on the same principle of the former, but applied to Wild Fowl, would be equally successful; and to this end the Committee recommend the passing of such a Bill as was introduced by Mr. Andrew Johnson in 1872, This Bill, it will be remembered, was the foundation of the existing · Wild-Birds Protection Act, but was so entirely altered in its passage through Parliament as to bocome useless for the protection of the group of birds it was at first intended to protect.
“ The • Wild-Birds Protection Act' may well be left as it is, since public opinion was, and is, decidedly in favour of some such legislation. Its failing to protect Wild Fowl efficiently gives no room for its repcal; but the Cominittee regards it as being virtually ineffective to produce any practical good.
“ The Committee thinks it necessary to state once more, that of the Small Birds which so deeply engage the sympathies of many of the public, thero are but few kinds which have been proved, on any good evidence, to be diminishing in numbers, and that the decrease of these is owing much less to any direct destruction or persecution than to indirect causes, such as have been already referred to, and declared to be uncontrollable by the legislature.
The diminution of such birds as the Wheatear, the Goldfinch, and Linnet can be immediately traced to the breaking up, and bringing under cultivation, of commons, and so probably of the rest; while, on the other hand, it is obvious that many kinds of Small Birds have largely increased in number owing to the spread of plantations, and the security from molestation during the breeding-season they enjoy through the incessant attention given to the preservation of game.
“At the same time the Committee is of opinion that some steps for the Regulation of Bird-catchers might well be taken, with the approval not only of the general public, but of the better class of bird-catchers themselves ; and, should success attend its present attempt, the Committee would readily direct its efforts to that object.”
Your Committee has the gratification of reporting that the opposition which the Bill encountered in the House of Commons, though seriously intended, was happily overcome by the good management of Mr. Chaplin and his seconder, Mr. Rodwell, Q.C. A division was taken on the motion for the Second Reading, when the numbers against it were 13, and in its favour 337 -an almost absolute majority of the whole House.
In deference to certain objections which were raised in Committee, Mr. Chaplin consented to an alteration of the original draft Bill as regards the days when the proposed “ Close Time” should begin and end. Your Committee cannot wholly approve of this change; but as it does not affect the length of the season, the modification seems not to be very important, while Mr. Chaplin's adroit acceptance of it unquestionably saved the Bill.
No further alteration was made. The Bill, having passed the Commons, was kindly taken charge of in the Upper House by Lord Henniker, and finally received the Royal Assent on the 24th of July.
In congratulating all who have at heart the protection of indigenous animals in this happy result, your Committee desires to point out that their most sincere thanks are due to the nobleman and gentlemen already named, as well as to others who aided the passage of the Bill through both Houses, and, in particular, the efforts of Lord Walsingham deserve especial recognition.
With regard to the taking of any further steps, your Committee can only suggest the possibility, of something being done in the direction indicated by the last paragraph of the foregoing statement. The difficulties, however, in the way of passing any measure for the Regulation of Bird-catchers, which should be at once effectual and acceptable to Parliament, seem to be very great, and your Committee is not sanguine of the success of any immediato attempt to attain this end.
The Sea-Birds Preservation Act continues to work satisfactorily on the whole, though your Committee has reason to fear that its provisions have been disregarded in certain places. Some time has elapsed since any prosecution under it has taken place; and its enforcement in a few instances in the course of the next year may be needed to show that it cannot be violated with impunity. To this object your Committee, if reappointed, will give its attention; meanwhile it may be observed that the Act is very favourably regarded in most places, and that, by authority of its third section, the Secretary of State for the Home Department has, on the recommendation of the justices of the East Riding of York in Quarter Sessions assembled, extended the “ Close Time” on the coast of that county from the 1st to the 15th of August.
Your Committee respectfully urges its reappointment.