Rock Blasting / A Practical Treatise on the Means Employed in Blasting Rocks for Industrial Purposes

by George G. André

In America, there are two frictional exploders in common use. One, shown in [Fig. 37], is the invention of H. Julian Smith. The apparatus is enclosed in a wooden case about 1 foot square and 6 inches in depth. The handle is on the top of the case, and is turned horizontally. This handle is removable, as in Bornhardt’s machine. The cable wires having been attached to the terminals, the handle is turned forward a certain number of times to excite the electricity, and then turned a quarter of a revolution backward to discharge the condenser and to fire the blast. By this device, the necessity for a second aperture of communication with the inside is avoided, an important point in frictional machines, which are so readily affected by moisture. The aperture through which the axis of the plate passes, upon which axis the handle is fixed, is tightly closed by a stuffing-box. A leathern strap on one end of the case allows the machine to be easily carried. The weight of this exploder is under 10 lb.

The other exploder used is that designed by G. Mowbray. This machine, which is shown in [Fig. 38], is contained in a wooden barrel-shaped case, and is known as the “powder-keg” exploder, the form and dimensions of the case being those of a powder-keg. The action is similar to that of the machine last described. The cable wires having been attached to the terminals at one end of the keg, the handle at the other end is turned forward to excite the electricity, and the condenser is discharged by making a quarter turn backward, as in Smith’s machine. The handle is in this case also removable. The weight of the powder-keg exploder is about 26 lb.

Both of these machines are very extensively used, and good results are obtained from them. They stand well in a damp atmosphere, and do not quickly get out of order from the wearing of the rubbers. They are also, especially the former, very easily portable.

Fig. 39.

The machine commonly used in England is the dynamo-electric exploder of the Messrs. Siemens. This machine, which is the best of its kind yet introduced for blasting purposes, is not more than half the size of Bornhardt’s frictional exploder; but it greatly exceeds the latter in weight, that of Siemens’ being about 55 lb. The apparatus, which is contained within the casing shown in [Fig. 39], consists of an ordinary Siemens’ armature, which is made, by turning the handle, to revolve between the poles of an electro-magnet. The coils of the electro-magnet are in circuit with the wire of the armature; the residual magnetism of the electro-magnet cores excites, at first, weak currents; these pass into the coils, thereby increasing the magnetism of the cores, and inducing still stronger currents in the armature wire, to the limit of magnetic saturation of the iron cores of the electro-magnets. By the automatic action of the machine, this powerful current is, at every second turn of the handle, sent into the cables leading to the fuses.

To fire this machine, the handle is turned gently till a click is heard from the inside, indicating that the handle is in the right position to start from. The cable wires are then attached to the terminals, and the handle is turned quickly, but steadily. At the completion of the second revolution, the current is sent off into line, as it is termed, that is, the current passes out through the cables and the fuses. As in the case of the frictional machines, the handle is, for safety, made removable. This exploder is practically unaffected by moisture, and it is not liable to get out of order from wear.

Induction coils have been used to fire tension fuses; but it is surprising that they have not been more extensively applied to that purpose. A coil designed for the work required of it is a very effective instrument. If constructed to give a spark not exceeding three inches in length, with comparatively thick wire for quantity, it makes a very powerful exploder. An objection to its use is the necessity for a battery. But a few bichromate of potash cells, provided with spiral springs to hold the zincs out of the liquid, and designed to be set in action by simply pressing down the zincs, give but little trouble, so that the objection is not a serious one. The writer has used an induction exploder in ordinary mining operations without experiencing any difficulty or inconvenience. It is cheap, easily portable, and constant in its action.

Batteries are used to fire what are known as “quantity” or “low tension” fuses. Any cells may be applied to this purpose; but they are not all equally suitable. A firing battery should require but little attention, and should remain in working order for a long time. These conditions are satisfactorily fulfilled by only two cells, namely, the Léclanché and the Bichromate of Potash. The latter is the more powerful, and generally the more suitable. The Léclanché is much used in this country for firing purposes, under the form known as the “Silvertown Firing Battery.” This battery consists of a rectangular teak box, containing ten cells. Two, or more, of these may be joined up together when great power is required. In France, the battery used generally for firing is the Bichromate. This battery is much more powerful than the Léclanché, and as no action goes on when the zincs are lifted out of the liquid, it is equally durable. It is moreover much cheaper. At the suggestion of the writer, Mr. Apps, of the Strand, London, has constructed a bichromate firing battery of very great power. It is contained in a box of smaller dimension than the 10-cell Silvertown. The firing is effected by simply lowering the zincs, which rise again automatically out of the liquid, so that there is no danger of the battery exhausting itself by continuous action in case of neglect. Externally, this battery, like the Silvertown, appears a simple rectangular box, so that no illustration is needed. With either of these, the usual objections urged against the employment of batteries, on the ground of the trouble involved in keeping them in order, and their liability to be injured by ignorant or careless handling, do not apply, or at least apply in only a very unimportant degree.