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field; but this would have increased the internal resistance, and would also have made the machine.

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Fig. XIV

much less efficient at light loads. method of regulation, we

By the present

reduced the C2 Rat

losses at one-quarter load from 4,018 to 3.367 watts; or, in other words, we gain almost one electrical horse-power.

Fig. XII is a curve of the electrical efficiency. It will be noticed that this at full load reaches ninety-four per cent, which is accounted for by the liberal allowance of iron in the armature, thus reducing the reluctance of the magnetic circuit.and by the large size of the wire used on field and armature.

on both

Fig. XIII is a curve of the commercial efficiency. At full load this is over ninety per cent, cent, and approaches very closely the efficiency of incandescent dynamos of equal capacity; but the most noteworthy point is the high efficiency shown at

one-quarter load.


Fig. XIV is a curve of the machine separately excited, with no current in the armature. The ordinates are volts at the armature terminals, and the abscissæ, the amperes in the field. This is really a permeability curve of the magnetic circuit. By comparison of the voltage shown here, when there are nine amperes in the field, with that of the machine when delivering current, can be seen the enormous armature reaction. The curve also indicates a new departure in arc dynamo design, namely, that the magnetic current is not worked at nearly so high a point of saturation as in the old. types.


MR. HAMILL: I, naturally, was very much interested in Mr. Black's paper. A man who sat for a number of years at the feet of such men as Professor Brackett of Princeton and Mr. Brush

of Cleveland, ought to be able to write a pretty good paper. My interest in these large dynamos is purely from the standpoint of the manufacturer and seller. During the past year the Brush Company has built large arc dynamos, from 8,000 to I0,000lights capacity. The inquiries concerning them do not come from new companies, but almost entirely from existing stations. This is natural, for the reason that nearly all our large electric light stations are quite centrally located in the large cities of the country, where real estate is expensive, where the business is growing, and where it is a question of either going further away from the centre of the cities, or purchasing additional real estate at great expense in order to increase the capacity, or exchanging the small dynamos for larger ones. I have in my mind, for instance, one of the largest and best managed local companies in the country, where they have two large stations quite near each other in the centre of a large city,-stations that have been in existence thirteen or fourteen yearswhere last year they had 2,000-lights capacity and arc dynamos ranging from old ten-lighters up to sixty-lighters, using a tremendous amount of shafting and belting. Naturally, these dynamos were very inefficient. This company exchanged all their old dynamos, small and large, for a complete new outfit of twelve or thirteen large dynamos; and by so doing they obtained a great deal of space in their existing stations for increasing their business, without going to the expense of enlarging.

I have in mind another station where they had almost exclusively fifty to sixty-light arc dynamos, with a capacity of about 2,000 arc lights. This station changed from fifty and sixty-light dynamos to 125

light dynamos. They saved a large amount of floor space, and increased the efficiency of their machines, which is a very telling and vital point. The field, so far as manufacturers are concerned, is at the present time almost entirely along the line of exchanges from the small to the large units, and there is very little profit in it for them.

PROFESSOR THOMSON: I have been quite interested in listening to the paper, as I consider it a very true exposition of some points connected with the operation and design of arc dynamos. The development of the arc dynamo, it is true, has been somewhat backward; still, I do recollect that machines were built far back in the early days which were capable of giving a curve similar to that which has been

shown a characteristic curve with an almost vertical drop; but they were disadvantageous from the very large armature reaction, the size of the armature, the amount of wire, and the dead resistance of the wire used in the construction.

The question as to whether open coil dynamos or closed coil dynamos are the best for arc lighting has been fully discussed in the literature of the subject for a long time. Considered purely as an electrical affair, without reference to the practical uses, there can be no question but that the closed circuit dynamo is the better electrical structure. I say, considering it as a dynamo; but when you come to use that dynamo under the conditions of practice, there may be considerations that will modify that conclusion. I am really sorry that Mr. Wood, the champion of the closed circuit dynamo, is not here. We might have had a discussion bringing out the strong points of

the other side.

When I first entered the business, my object was

to get the kind of machine that, put into ordinary hands,-men of no experience with machines of the kind-could be run successfully and keep the lights going; and the object was to reduce to the utmost simplicity those parts that were subject to wear and that could be injured by bad handling. Therefore, the commutator was reduced to a very few parts, and it was so constructed that if anything happened to it to injure the segments, it was a simple thing to take off the segments and put on others. That could not be done with a commutator containing from 100 to 200 segments. In that case, it would mean that you would have to take out the whole commutator, and lay it aside and get a new one, and make all the connections before you could start the machine. In the early history of dynamos, we did not have experienced men to handle them. The dynamos were not put in stations with a dynamo tender of experience in charge. They were in charge of an engineer, who might know how to run his engine, but knew nothing about the dynamo, and who had a superstitious idea that that was a thing which he must not touch. We have had enough cases such as this arise. case, the brushes on the dynamo did not rest on the commutator, and we had a communication that something was the matter with the machine and we were to send the best expert we had, right along. The moment we go into a new field, we have to educate men to understand how to handle the new structure, on account of its peculiarities. The fact that an open coil dynamo, when it has its circuit interrupted, will not give the rise of voltage that the closed coil dynamo gives, has been known for a long time.

In one

The point of flashing was admirably brought out

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