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

by George G. André

—The electrical machines used for firing tension fuses are of two kinds. In one kind, the electricity is excited by friction, and stored in a condenser to be afterwards discharged by suitable means provided for the purpose. In the other kind, the electricity is excited by the motion of an armature before the poles of a magnet. The former kind are called “frictional electric” exploders; the latter kind are known as “magneto-electric” exploders. When a magneto-electric machine contains an electro-magnet instead of a permanent magnet, it is described as a “dynamo-electric exploder.”

Frictional machines act very well as exploders so long as they are kept in a proper state. But as they are injuriously affected by a moist atmosphere, and weaken rapidly with use by reason of the wearing away of the rubbers, it is necessary to take care that they be in good electrical condition before using them for firing. Unless this care be taken, the quantity of electricity excited by a given number of revolutions of the plate will be very variable, and vexatious failures will ensue. If, however, the proper precautions be observed, very certain and satisfactory results may be obtained. In Germany and in America, frictional exploders are generally used.

Magneto-electric machines possess the very valuable quality of constancy. They are unaffected, in any appreciable degree, by atmospheric changes, and they are not subject to wear. These qualities are of inestimable worth in an exploder used for ordinary blasting operations. Moreover, as they give electricity of a lower tension than the frictional machines, defects of insulation are less important. Of these machines, only the dynamo variety are suitable for industrial blasting. It is of primary importance that an exploder should possess great power. The mistake of using weak machines has done more than anything else to hinder the adoption of electrical firing in this country.

Fig. 36.

The machine most used in Germany is Bornhardt’s frictional exploder, shown in [Fig. 36]. This machine is contained in a wooden case 20 inches in length, 7 inches in breadth, and 14 inches in depth, outside measurement. The weight is about 20 lb.

To fire the charges by means of this exploder, the leading wire is attached to the upper terminal B, and the return to the lower terminal C, the other ends of these wires being connected to the fuses. The handle is then turned briskly from fifteen to thirty times, according to the number of the fuses and the state of the machine, to excite the electricity. The knob A is then pressed suddenly in, and the discharge takes place. To ascertain the condition of the machine, a scale of fifteen brass-headed nails is provided on the outside, which scale may be put in communication with the poles B and C by means of brass chains, as shown in the drawing. If after twelve or fourteen turns, the spark leaps the scale when the knob is pushed in, the machine is in a sufficiently good working condition. To give security to the men engaged, the handle is designed to be taken off when the machine is not in actual use; and the end of the machine into which the cable wires are led is made to close with a lid and lock, the key of which should be always in the possession of the man in charge of the firing operations.

Fig. 37.

Fig. 38.