of the plates which is in the water, and the slip can be varied from 20 per cent. down to 6 per cent. by the small slip plates which are illustrated at the top of the main plate. We are running at between 7 and 8 per cent. slip.

K. A. PAULY.-The efficiency, as determined from the power consumption and work done with the hoist, seemed to vary from 57 per cent., over all, to 49 per cent. I would like to ask how much of this 43 per cent. and 51 per cent. of loss is due to shaft loss and mechanical parts, and how much from the hoist coupled to the main motor, if that is known.

F. H. ARMSTRONG. That is not known.

BENJAMIN F. TILLSON, Franklin Furnace, N. J.-Speaking of the signaling system, I notice it is spoken of as a grounded system. I would ask if any trouble has occurred through what you might call "spook signals," from inadvertent grounding in other parts of the system, or other parts of the power plant. In our alternating-current system at Franklin Furnace, we have occasionally had some "spook signals," although our system is not supposed to be a grounded system, due to the grounding of a line, which was unnoted until such signals were given. Our voltage will run somewhere around 25 or 30 volts through the bell magnets.

F. H. ARMSTRONG.-Our voltage is about the same. noticed anything of the kind.

We have never

J. E. JOHNSON, JR., New York, N. Y.-I would ask how the efficiency of these electric hoists, of which we have been hearing, compares with the efficiency of the compressed-air hoist system which was applied at Anaconda. The Anaconda Co. put that job up to the electric companies of the country, and they could not come up to the specifications. Then Mr. Nordberg put in the compressed-air system, which gives an efficiency of somewhere upward of 50 per cent., and is giving the most absolute satisfaction, with a minimum of danger and a maximum of convenience. If we could get hold of some information as to the relative efficiencies of these electrical installations and the compressed-air system put in by Mr. Nordberg, of which there is given a full description in a recent issue of the Bulletin, it would be very illuminating.

WILLIAM KELLY, Vulcan, Mich.-That information can be had in large part now by all who are interested. In addition to the paper by B. V. Nordberg in the Bulletin, there have been other papers on the subject, one by D. B. Rushmore and K. A. Pauly, presented at a meeting of the American Institute of Electrical Engineers on Mar. 11,

We expect the papers now under discussion will be followed by another paper, to be presented a little later, on the results of electric hoisting at a Lake Superior mine, and when all these papers are brought

together and compared, the comparative costs by the different methods of hoisting can be arrived at quite closely.

K. A. PAULY.-I would like to ask the gentleman who mentioned the 50 per cent. efficiency for the air hoist where he obtained that figure. In the first place, I do not believe that, theoretically, such a figure is possible. In the second place, I have seen the results of the tests made at Butte, and they do not give over 29.5 per cent.; and, on top of all of that, efficiency in an air system means nothing anyway, because you have to pay for reheating the air. You have a much higher maintenance expense, both on the distributing system and the actual operating part of the equipment, the air engine.

Unquestionably the Butte system has been a complete success from an operating standpoint during the period that it has been installed, but I think that all of those who are connected with mining operations will admit that one of the most troublesome pieces of machinery about a mine is the air end of the air compressor. I think we may look for the same trouble on the engine end of the air compressor, after it has been installed some few years.

J. E. JOHNSON, JR.-The figures which were published were given by Mr. Nordberg. They include the power coming in on the line to the transmission line, and the load hoisted on the shaft, which is the over-all efficiency, if there is any such thing, and these figures are about 50 per cent.

I do not know where this gentleman gets his data of 29 per cent., but I do know that papers have been published, concerning the electrification of mines and central hoisting systems using electricity, that have described travesties on good engineering. They have spent twice as much money in the installation of these systems as a compressed-air system would have cost, and have given no better result than the compressed-air system, in some cases not as good. When the gentleman says that the air end of a good air compressor will create trouble, as compared with the electric motor, I can only say that he has had an unusual experience. I have had a good deal of experience with both, and will back the air end of the air compressor to let a man sleep a good many more nights in a year than the electric motor will.

Electric Traction in Mines

BY CHARLES LEGRAND, DOUGLAS, ARIZ.

(New York Meeting, February, 1914)

In many iron, coal and copper mines where large tonnages are known before starting operation and proper provisions can be made, the problems of electric traction by trolley locomotives are not very different from those of surface plants. In such installations the gauge of the track, the radius of curves, and the clearances, both vertical and horizontal, can be made to suit the conditions of the traffic. It is more difficult to install electric traction in mines which were started with hand tramming and where no consideration was given to the possibility of mechanical traction being used. The writer having had some experience in the installation of electric traction in copper mines, the following remarks apply more particularly to these mines.

With the gauge of track usually 18 or 20 in., the weight limit of locomotives obtainable from manufacturers in this country varies from 3 to 6 tons. The full-load speed varies from 4 to 6 miles per hour. These locomotives, being made to run on very small radius curves, have a short wheel base and a long overhang from axle to coupling, which necessitates a coupling with a good deal of lateral motion to avoid derailing the cars on sharp curves, especially if couplings are of the standard railroad automatic type.

Although 3-ton locomotives will run on 12- or 16-lb. rails, it has been found more satisfactory to use 25-lb. rails, as the track keeps in much better shape, it is easier to maintain the bonding in good order, and fewer derailments from dirt on the track occur with the larger rails. Where 6-ton locomotives are used the 25-lb. rails are satisfactory, but 40-lb. rails have proved cheaper where the traffic is heavy and the ground is soft, as the track maintenance is considerably lower with the heavy rails. The locomotives will run on 15-ft. radius curves, but on through runs it is advisable not to go below 40-ft. radius.

The voltage used should not exceed 250 to 275 volts, and the trolley wire should be protected, to prevent accidental contacts, in front of chutes and at all points where it is low. With the air lacking somewhat in oxygen and the heat and high humidity prevalent in many mines, this

voltage, which is considered perfectly safe, has proved fatal in several instances and in such mines it is advisable to have a pulmotor available and men trained to use it in case of accident.

The trolley wire should be protected from dripping water and if the water is acid it must be protected; the writer has seen instances where a very small drip has cut a No. 00 trolley wire in less than three weeks.

The track bonding should be kept in good shape. This is one of the most difficult things to do, as most of the trackmen in the mines do not realize the importance of it. A badly bonded track will increase the repairs of motors considerably, as in passing from a dead rail to a live one the sudden rush of current is liable to form an arc across the motor commutator or from the commutator to the ground.

If the locomotive is not mounted on springs it has been found advisable to put the resistance grids on springs, with flexible leads to the controller, as on small locomotives the cast metal grids are light and very easily broken.

In a mine laid out for hand tramming the grade is generally made in favor of the loaded cars (this is also done to provide drainage for the mine), so that the load is fairly uniform going down with the loaded cars and coming up with the empty cars. This gives ideal conditions for a full load on locomotives at all times; but the motors on electric locomotives are seldom made so that the locomotive can deliver its full tractive effort continuously and this ideal operating condition leads to overheating of the motors and very heavy repairs unless the number of cars attached to the locomotive is kept down to the maximum that the motors can pull without overheating. This is difficult in practice, as it seems against human nature to run a locomotive of any kind with a load that does not slip the driving wheels when starting or at every point in the track where conditions are a little unfavorable. The difficulty of getting motors of sufficient size in the small space available with 20-in. gauge has obliged us in one or two instances where traffic is heavy and continuous to build our own locomotives, putting the motors above the wheels and gearing to the axles outside of the wheels. This makes a rather cumbersome design but allows the use of larger motors and has proved satisfactory in service. Even with this design the necessary clearance in the drifts limits the weight of locomotives to about 7 tons.

Where the tonnage to be handled is not great and yet mechanical traction is advisable a storage-battery locomotive is convenient. The running expenses are not much greater than with a trolley system, depending on the conditions under which the locomotive has to operate.

The Copper Queen Mining Co. operated a 3-ton storage-battery locomotive at Bisbee for over two years, under the worst conditions of any locomotive in their mines as regards track and curvature, and the results were better than the writer anticipated. To make use of one of the

regular locomotives, the battery, consisting of 150 Edison cells, was mounted on a separate trailer. This battery had a total output capacity of 40 kw-hr., the average voltage on discharge being 180 volts. The first trays furnished to hold the cells were of the regular type for automobiles and proved to have too small a clearance between cells. The hard bumping in switching combined with very heavy sweating (due to the locomotive going from very hot portions of the mine with moisturesaturated atmosphere to colder portions near the shaft) short circuited the cells externally. After the trays were altered to provide larger clearances and the cells were painted with insulating paint there was no trouble from this source, although two or three cells were lost in a bad wreck.

The power required at power station per useful ton-mile was approximately double that required with trolley locomotives, or 1.6 kw-hr., due to extra dead weight of battery car and lower efficiency of battery compared to trolley wire and track circuit, also to the losses in the motor generator used in charging the battery. With a locomotive designed to carry batteries the difference in power would be less. The power would also have been reduced if a motor controller had been used, grouping the cells in various combinations for starting, instead of a regular controller with starting resistances.

The traffic got too heavy to be handled with this locomotive and it did not run long enough to get figures on depreciation of storage battery. The maintenance of the battery-locomotive motor was less than on the trolley locomotive, but no exact figures are available. The capacity of the battery was approximately 50 useful ton-miles on one charge.

At the mines of the Copper Queen Mining Co., in Bisbee, the power used on trolley locomotives, measured at direct-current switchboard in power station, for the year 1912 amounted to 875 watt-hours per useful ton-mile. This amount, however, includes a few lights which are connected to the trolley circuit and gives too high a figure for the locomotives alone. It applies to cars with roller bearings, about one-half of the tonnage being carried in cars of 2 tons capacity and the other half in cars of 1 ton capacity. The conditions of the cars and track have quite an important bearing on power required per ton-mile, although the writer has no accurate figures. A rough idea can be formed from the fact that on a certain track in the mine of the Moctezuma Copper Co. one 3-ton locomotive cannot pull more than five cars of 20 cu. ft. capacity, equipped with regular Anaconda axles, without slipping the wheels, while the same locomotive pulls six cars of 22 cu. ft. capacity equipped with rollerbearing axles.

For the year 1912 the cost of various items in cents per useful tonmile at Bisbee for a total of 408,000 ton-miles was as follows: