Page images
PDF
EPUB

heat flowed through the pile. By passing hot or cold water through holes in the pole-pieces we were able to alter the flow at will. The following are the results obtained :—

[blocks in formation]

These numbers are each the mean of ten determinations, and show clearly the alteration of sign from - to as the flow of heat changed from the iron-pile to the pile-iron direction. They accord with the sign of a set of readings we took, in which the end of the pile remote from the iron was warmed by a tube carrying a stream of tepid water (curve ƒ); and they also explain the change of sign which occurred while the paraffin slab was cooling (p. 479).

Having thus traced our first results to the unsymmetrical arrangement of the pile, the obvious course was to put it in such a position that the flow of heat through it was avoided altogether, and to repeat our original readings. This we tried to do, but without success. It was easy to reduce the flow to a very small amount, but quite impossible to keep it so for more than a few minutes at a time. We venture to think, however, that the curves obtained are sufficient to prove the absence of the molecular effect sought for. A comparison of the curve for artificial heating with the others shows that the peak, which is so marked a feature in the former, is not found in the latter. It is true that if only the normal curves b, c, d, e are considered, it might be urged that the peak is perhaps absent because it is neutralized by an equal and opposite peak in the effect which gives the readings their negative sign. But if this were so, it should be doubly present in the curves

a and f, where the two peaks would be in the same direction. That this is not the case seems to us to render it practically certain that it does not exist at all*. In other words, that if the molecular currents are due to a spinning motion of ionic charges, this motion is not inseparably connected with a spinning of the molecules themselves.

LVII. A New Method of Magnetizing and Astaticizing Galvanometer-Needles. By F. L. O. WADSWORTH t.

[ocr errors]

[Plate XIII.]

GREAT deal of attention has been devoted by many eminent physicists to the improvement of the galvanometer, and it has in consequence been brought to perhaps as high a degree of delicacy and excellence as any of our standard physical instruments. It is vain to hope for any great advance in sensitiveness in existing types, but there will always be a steady improvement in details and methods of construction.

One of the first essentials in securing a maximum degree of sensitiveness in a galvanometer of the Thomson type is to secure the maximum intensity of magnetization in the magnetic system. The maximum attainable will depend on three factors-first, on the form of the individual magnets and their arrangement with reference to each other; second, on the quality of the steel used in making the magnets, and the method of hardening and tempering the same; and third, on the method of magnetization. I have found that the intensity of magnetization, and hence the sensitiveness of the galvanometer, depends on this last factor to a greater degree than has usually been supposed, and that by using the method which will presently be described the sensitiveness of a given galvanometer may be doubled and in some cases more than quadrupled. This results not so much from an improvement in the magnetizing process per se as from the method of application.

The usual method of making the astatic system of the Thomson galvanometer is to build up each member of the system from a number of individual bar-magnets, each of which is hardened and magnetized as strongly as possible between the poles of a powerful electromagnet, either during

*The only way out of this conclusion is to suppose that the peak has been neutralized by some effect which we have overlooked, and which does not change its sign with the direction of the heat-flow. But this is unlikely.

† Communicated by the Author,

the process of hardening and tempering or subsequently*. During the process of building up, the individual magnets are subjected to various demagnetizing influences, the most serious of which are the jars and blows received in the process of mounting, and the proximity of other magnets during the process of astaticizing. The resultant strength of each member of the finished system is therefore considerably less than it would be if the individual magnets were magnetized to their saturation-point. Prof. Threlfall † avoided this difficulty by magnetizing each member of the system in situ, using for this purpose two separate electromagnets, one for each member.

It was this which first suggested to me the present method, which differs from the preceding in that but one electromagnet is used for simultaneously magnetizing both members of the system. The advantage of this is that both members, when being magnetized, form part of one and the same magnetic circuit, and hence, barring magnetic leakage (which may be reduced to a minimum by proper design of circuit), are necessarily in fields of precisely the same strength for all magnetizing forces-one essential condition for securing astaticism, or in preserving it when once established and remagnetization is desirable. When separate electromagnets are used for each member, it is necessary in order to secure this condition that the field of each be of exactly the same strength, an end which requires for its accomplishment the taking of unusual precautions (described in the paper referred to) in the selection of material, the winding of the coils, &c. The single electromagnet device is therefore considerably simpler to construct and more convenient to use.

[ocr errors][merged small][merged small]

Fig. 1 is an elevation of the electromagnet in the form in * Simultaneous magnetization and hardening does not seem to possess any advantages over the ordinary method. See Holz, Wied. Ann. vii. (1879). "Measurement of High Resistance," Phil. Mag. vol. xxviii. p. 452

(1889),

which it was first used. It consists simply of two pieces of round iron inch diameter, each bent into the shape of a very shallow U, with legs about 1 inch long and back or yoke 22 inches long (this being the distance between the centres of the two members of the astatic system), wound with about 350 turns of No. 18 magnet-wire. With a current of 5 amperes in the magnetizing coils, and a separation between the poles of the two halves of 3 millim., the density in the iron cores amounts to about 5000 C.G.S.

The system to be magnetized is laid on its back on the edge of a narrow strip of wood b mounted on a board B (figs. 1 & 2, Plate XIII.). This strip is a very little wider than the length of the needles of the system, and of such a height that the top edge is just below the centre of the poles of the electromagnet, the two yokes of which rest on the supporting base. Thin strips of glass or mica are cemented on each side of the central strip opposite the poles of the magnet, to prevent the latter from touching the needles. The coils of the two yokes are so connected that the whole forms a single magnetic circuit with the two air-gaps m, n, in each of which lies one of the members of the magnetic system, so that when the current is turned on by means of a key, K, they are strongly and equally magnetized in opposite directions, as required. In order to secure the maximum permanent magnetization, the strength of the field should be suddenly increased to a maximum and then gradually decreased, several times in succession, either by varying the strength of the current in the coils by means of a rheostat, or, better, by varying the distance between the two poles*, tapping the support with a light hammer or block of wood when the magnetic induction is at a maximum, in order to assist in "setting" the molecular structure. The magnetization being finished, the system is lifted carefully from its support, hung by means of its attached fibre in a glass tube large enough to allow it to swing freely, and tested for astaticism.

* In no case should the magnetizing current be broken suddenly when the poles are close to the needles, or the strength of the latter may be very considerably weakened, if not destroyed, by the sudden recoil of the field. See Auerbach, Wied. Ann. xiv. (1881), and Fromme, Wied. Ann. v. (1878):--"Auerbach lays down as a general principle that when the variation of the magnetizing force is slow and continuous the velocity of the transition does not influence the final magnetization; but sudden transition causes the final magnetization to be less or greater than that obtained by gradual transition, according as the passage is from a greater to a less or from a less to a greater force." (Encyc. Brit. art. "Magnetism," p. 260.)

In this connexion see also paper by Lord Rayleigh, Phil. Mag. [4] vols. xxxviii, & xxxix. (1869, 1870),

Usually, owing to difficulties of construction, one member will be slightly heavier and hence magnetically stronger, and the magnetic planes will be slightly inclined to each other; the amount and direction of which inclination can be ascertained by noting the setting of the system with reference to the magnetic meridian.

S

Fig. 2.

N

The planes of the members should first be brought to parallelism by a slight twisting of one of them on its support. A good way to do this is to take two bar-magnets, N, S (fig. 2), and bring first one and then the other up against the side of the glass tube, as in fig. 2, opposite the magnetically stronger of the two members, which will be drawn against the wall of the glass tube toward the magnet first brought up, say N. If a right-handed twist is needed to bring the two p'anes to parallelism, the two magnets are held as shown, in front of the axis of the tube; if a left-handed one, behind the axis, as indicated by the dotted lines. Then the magnet N is quickly removed and the member is drawn toward S against the inclined wall of the tube, giving it a blow which tends to twist it clockwise on the axis of suspension and at the same time to slightly weaken it. Repeating this operation a few times will soon suffice to bring about the required degree of parallelism unless the cement which attaches the member to the staff be too hard and dry, in which case a slight heating of the surrounding tube with a hot block of copper or a Bunsen-flame will be necessary to render the cement slightly plastic.

Parallelism having been secured (which is determined by the plane of the system lying in the magnetic meridian), the relative magnetic strengths of the two members are tested by determining the time of vibration. Generally, if the system has been made up with a proper degree of care, the strengths of the two members will be so nearly equal that it will only require a very weak directing magnet placed some distance above or below the system to secure almost perfect astaticism.

But if it is desirable to secure this without the use of such a magnet, it will be necessary to add to the weaker of the two systems a minute fragment of steel (a very short piece broken off from one of the finest watch hair-springs answers well), and then to remagnetize the whole as before, taking care of course to magnetize in the same direction. The whole operation requires at the most but a few hours' work by an experienced manipulator, and the success of this method may be judged by the results which have been obtained by its use.

« PreviousContinue »