But we must not forget that such qualities are purely theoretical. In practice the best machine is that in which the copper is best utilized, that is to say, that which with a given weight of this metal furnishes the most work. Now, this is certainly not the case in the Pfaundler machine, for here half or more than half of the ring is inert—a defect which is apparent at first sight. It results from this that as soon as we propose to obtain an electromotive force, however slight it be, we must get it with machines of large dimensions. Now, it is permissible to believe that under such circumstances (taking into consideration the complication of mechanical means that the construction of such apparatus necessitates, and the great friction that occurs) it would be impossible to obtain practical rotary velocities. Comparing his machine with Gramme’s, Prof. Pfaundler expresses the idea that between them there is the same analogy as there is between a constant pressure and an expansion engine. With cylinders of equal diameters the work performed by the former of these is greater than that done by the second, but in the latter the expansive force of the steam is better utilized. This comparison seems to us to be more ingenious than exact. Would it not be coming nearer to the truth if we were to suppose a case of a hydraulic motor whose performance continued diminishing with the height of the fall, and would it not be advantageous under such circumstances to utilize only a portion of the fall for the purpose of increasing the motor’s performance?
This machine, however, as before stated, has never as yet been constructed, so that experimental data relative to its mode of working are wanting. It is especially interesting as regards its origin, which dates back to an epoch at which researches on the dynamo electric machine were at their heat. It is in its historical aspect that it is proper to regard it, and it is from such a point of view that we have deemed it well to say a few words about it in this place.—La Lumiere Electrique.
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We shall not attempt to pass in review the several apparatus that have hitherto been devised for igniting blasts in mining operations, but shall simply describe in this place a machine recently invented for this purpose by Mr. Bornhardt, an engineer to the Grand Duke of Brunswick.
This apparatus (shown in the accompanying engravings) consists essentially of two hard-rubber disks, A (Figs. 2 and 3), keyed to an iron axle, and of two rubbers, B, that are formed of skin and are held against the disks by small springs, R; motion is communicated to the axle, a, by means of a pair of gearings, a and b, and a crank, f.
[Illustration: BORNHARDT’S ELECTRIC MACHINE FOR BLASTING IN MINES.]