ELECTRICITY IN MINING 45 



being able apparently to do the same work with a consump- 

 tion of about one tenth of the power. Moreover, the electric, 

 drill does not sustain the further loss in efficiency to which 

 an air compressing plant is subject at a high altitude, nor is 

 it affected by a low temperature, which in an air compressing 

 plant freezes up the exhaust. 



A report has recently been made in some detail with re- 

 gard to results obtained with electric rotary drills as com- 

 pared with air drills in some potash mines in central Europe. 

 For the electrical equipment current is distributed through 

 the mine by armored cables fastened to the roof. The motor 

 is furnished with a reversing arrangement, such that two wires 

 of the cable can be interchanged; by this means the rotation 

 field of the motor, and therefore the direction of rotation of the 

 motor, is reversed. The motor has a starting resistance; it 

 develops normally about 1 horsepower, but can be overloaded 

 with 1.5 horsepower for a certain length of time. The con- 

 nection between drill and motor is made by a flexible shaft. 

 The weight of the drill proper, which is fastened between two 

 screw columns, is 99 pounds. The feed has been varied from 

 4 to 20 inches in one minute. 



In coal mining operations the use of electric rotary drills 

 and augers is quite common, and appears to be thoroughly 

 successful from the fact that the rotary motion of the motor 

 armature can be fully availed of. These drills are used for 

 boring holes into the coal for shooting it down after it has been 

 undercut. Many admirable t>^es of these drills are in wide 

 use. They are so arranged that they can be operated at 

 almost any angle, vertically or horizontally. They are gen- 

 erally mounted on light upright stands, with screws at the 

 ends for fastening to the roof and floor, but some of the larger 

 drills are mounted on trucks, so that they can be transported 

 readily from place to place. The motors, which often are 

 very small and light, not much heavier than a good sized fan 

 motor, are mounted with an adjustable clamp, and are geared 

 for either single reduction or double reduction, for reducing 

 the high speed of the armature to that which is suitable for 

 the auger. The larger rotary drills of this class have been 

 equipped with motors of from 4 to 6 horsepower, the power 



