We have received from M. Menges (of the Hague) a most interesting description of an apparatus on which he has been at work for some time past, with the object of generating electricity by the direct conversion of heat, or, as it might be more accurately described, by a more direct conversion than that of an ordinary dynamo. M. Menges' apparatus depends, like that of Edison, upon the fact that the magnetic metals lose their magnetic permeability at a certain temperature.
It differs greatly, however, from its predecessor in important points, especially in the fact that it does not require the aid of any external source of motive power.
In Edison's pyromagnetic dynamo it will be remembered that it is necessary to provide some small amount of motive power from an extraneous source in order to revolve the shield by which the heat is alternately directed on one half or the other of the armature cores. M. Menges' apparatus is, on the contrary, wholly automatic.
We proceed to give a free translation of the description furnished us by the inventor.
In attempting to employ the thermo-magnetic properties of iron or nickel in the construction of machines for the generation of electricity upon an industrial scale, we are met with the difficulty that the heating and cooling of large masses of metal not only involves great loss of heat, but also requires much time. Hence, to obtain a useful effect of any importance, it would appear necessary to employ machines of dimensions altogether impracticable. By the device and method of construction now to be explained this difficulty has, however, been completely overcome.
The action of a magnetic pole diminishes so rapidly with the increase of distance that it may suffice to remove the armature to a distance relatively small compared with its own dimensions, or with those of the magnet, in order to reduce the action to a negligible value. But if the magnet, N S, and the armature, A, being at a certain distance, we bring between them a piece of iron or nickel, d, then the magnetic force upon A is immediately and very considerably increased. In modern language, the resistance of the magnetic circuit has been reduced by the introduction of a better magnetic conductor, and the number of lines of force passing through A is proportionately increased. The mass of the piece, d, may, moreover, be relatively small compared with that of N S and A. If d be again withdrawn, the magnetic resistance is increased, and the lines through A are again a minimum.
Now, it is evident that we can also obtain the same effect by sufficiently heating and cooling the intermediate piece, d; and again, with a broad field we can alter the distribution of the lines at will by heating or cooling one side of this piece or the other. For this reason we will call the piece d the thermo-magnetic distributor, or, briefly, the distributor.
We will now describe the manner in which this principle has been realized in the practical construction of both a thermo-magnetic generator and motor.Fig. 1.
Fig. 1 shows an elevation and part section of one of the arrangements employed. Fig. 2 is a plan of the same machine (in the latter the ring, a a, appearing on a higher plane than it actually occupies).Fig. 2.
N S is an electro-magnet, a a the armature, wound as a Gramme ring, and fixed to a frame with four arms, which can turn freely upon a pivot midway between the poles. The cross arms of the frame are attached at 1, 2, 3, 4, Fig. 2. Between the magnets and the armature is placed the distributor, d d, where it occupies an annular space open above and below. Both the magnets and the armature are coated on the sides facing the distributor with mica or some other non-conductor of heat and electricity. The distributor is attached to and supported by the cross arms, so that it turns with the armature.
The distributor is composed of a ribbon of iron or nickel, bent into a continuous zigzag. This form has the advantage of presenting, in the cool part of the distributor, an almost direct road for the lines of force between the poles and the armature, thus diminishing the magnetic resistance as far as possible. At the same time the Foucault currents are minimized. To the same end it is useful to slit the ribbon, as in Fig. 3. This also facilitates the folding into zigzags.Fig. 3.
The distributor is heated at two opposite points on a diameter by the burners, b b, above which are the chimneys, e e. The cooling of the alternate section is aided by the circulation of cold air, which is effected by means of the draught in the chimneys, e e. At the points of lowest temperature a jet of air or water is maintained. The cross arms are insulated with mica or asbestos at the points where they extend from the armature to the distributor.
It will now be evident that while the distributor is entirely cool, many of the lines of force pass from N to S without entering the armature core; but if heat is applied at the points 1 and 2 in the figure, so as to increase the magnetic resistance at these points, then a great portion of the lines will leave the distributor, and pass through the armature core. Under these conditions, so long as heat is applied at two points equidistant from N and S, we might, if we so pleased, cause the armature to be rotated by an external source of power, and we should then have an E.M.F. generated in the armature coils - that is to say, the machine would work as an ordinary dynamo, and the power expended in driving the armature would be proportionate to the output.Fig. 4.
Suppose next that the points of heating, and with them the alternate points of cooling 90 deg. apart, are shifted round about 45 deg., so that the two hot regions are no longer symmetrically situated in respect to each pole of the field. The distribution of the magnetization has therefore become unsymmetrical, and the iron core is no longer in equilibrium in the magnetic field. We have, in fact, the conditions of Schwedoff's experiment upon a larger scale, and if the forces are sufficient to overcome the frictional resistance, a rotation of the ring ensues in the endeavor to restore equilibrium. The regions of heating and cooling being fixed in space, this rotation is continuous so long as the difference of temperature is maintained. The ring in rotating carries with it the armature coils, and of course an E.M.F. is generated in the same way as if the motive power came from an external source. In this respect the machine therefore resembles a motor generator, and the rotation is entirely automatic.
The armature coils are connected with a commutator in the usual way, and the field may, of course, be excited either in shunt or in series. M. Menges says that the residual magnetization is sufficient in his machine to start the rotation by itself.
When the machine is to be used as a motor, it is evident that the windings on the armature core need only be sufficient to supply current to excite the field, or by the use of permanent magnets they may be dispensed with altogether.
M. Menges has further designed a large number of variations on the original type, varying the arrangement of the several parts, and employing armatures and fields of many different types, such as are already in use for dynamos.
In Fig. 4 a machine is represented in which the field is external to the armature.
In Fig. 5 we have a thermo-magnetic generator, which corresponds to the disk machine in dynamos. Similar parts are indicated by the same letters in each of these figures, so that no further detailed description is necessary.Fig. 5.
In another modification M. Menges proposes to rotate the burners and leave the armature and distributor at rest. But in this case it is evident that the E.M.F. produced would be much less, because the magnetization of the core would only undergo a variation of intensity, and would nowhere be reversed, except, perhaps, just in front of the poles. In machines modeled on the Brush type it is evident that the distributor need not be continuous.
Enough has, however, been said to indicate the extent of the field upon which the principle may be applied. - The Electrician.