effect the time of opening a circuit has upon a typical power plant.

The heretofore standard form of magnetic circuitbreaker, and the type familiar to all, is immeasurably preferable to fuses, in so far as it opens the circuit much quicker on the score of time. Fuses open the circuit so late after the heavy unusual condition of circuit is established, that the current reaches the high and parallel part of current curves. For illustration, curve No. 8, in which we assume current has been flowing at, say, 100 amperes, with a 150 ampere fuse to protect the circuit, suppose a short circuit occurs, making a condition wherein 1,000 amperes would flow owing to ohmic resistance, if not checked.

The fuse would not blow until the current had at least reached point B on curve, or some point beyond. The ideal point to open the circuit should be as near the point A as would be possible.

While the regular well-known types of magnetic circuit-breakers open before fuses do, still they are not what they should be, when the heavier, more abnormal short circuits occur on circuits, for the reason that the magnetic circuit-breakers as at present constructed have practically an unvarying fixed time element; that is, they open in the same time whether the condition of circuit be such as to just permit the passage of current required to open the circuitbreaker, or the condition of circuit be such that the passage of current is due to a short circuit.

The opening of the magnetic circuit-breaker, as constructed, depends either on springs or gravity; the role the magnetic feature plays is simply to allow either the spring or gravity to act on the switch.

A magnetic circuit-breaker should be constructed to open the circuit in less and less time as the conditions become more and more severe. This can

be done by taking advantage of the flux of lines of force due to the passage of the current, since, fortunately, the two are simultaneous; and by designing the magnetic circuit-breaker to open as

regards time in an inverse ratio to the increase of the lines of force, and not depending on the springs of gravity, except, possibly, as auxiliaries to act as potential energy to aid in overcoming the inertia of the switch jaws and arms. We have then the correct method of protecting electric circuits.

The destruction of electrical apparatus, the breaking

down of insulation, the burning out of circuits, is taken by the business managers of electrical stations. with a meekness and a non-effort for relief which

is astonishing. What business manager would permit his boilers to run with safety-valves of such a type that an increase of 500 to 1,000 per cent of pressure would be required to open them?

The writer has repeatedly made tests showing that a No. 21 B. and S. gauge copper wire would permit the passage of 450 amperes for for 13 of

100

a second without burning out, without changing color. This is significant, as it shows that under conditions in practice the mechanical shock or torque tending to wrench or rupture insulation would be present in a marked degree, and further faults and flaws would develop which would by recurrence eventually cause the break-down in insulation. Owing to the sluggishness of fuses, permitting current to rise to the level point of current curve, wires under insulation in machines, as well as building wiring

unquestionably at times become red-hot. I have seen wires become heated momentarily red-hot, and cool off immediately after current was checked.

How many unknown, mysterious fires have started from this cause in electric wiring circuits?

All switchboards and minor lighting circuits should be protected by automatic (as regards time element) magnetic circuit-breakers, and the urgency of this will appeal to the exchequer of the management of our many electric light and power stations when the full meaning and import of Helmholtz's law dawns upon them.

On motion, the paper was received and ordered to be spread upon the minutes.

THE PRESIDENT: The next paper is by Mr. C. N. Black, on "Large Arc Dynamos."

LARGE ARC DYNAMOS.

Glancing back over the past fifteen years, and noting the rapid progress made in all branches of electrical engineering, it appears strange that arc dynamos have met with so few changes; more especially so when we remember that arc lighting was one of the first branches of the science reduced to practical use on a large scale. It is impossible to ascribe this to the fact that there was nothing left to be desired in the design or the efficiency of this type of machine, since it has always been notoriously weak in respect to efficiency, and has by no means kept pace with the incandescent dynamo. Within the past eighteen months, however, various manufacturers have awakened to this fact, and we have seen the results in numerous descriptions of new arc dynamos, appearing from time to time in the technical journals. The general tendency seems to be toward larger units and lower speeds; but, although the increase in size has, of course, increased the efficiency to some extent, we still notice the massive field magnets with their enormous amount of copper, and the small armature, which alone is enough to account for the inefficiency of these dynamos. Another noteworthy fact is that, with one exception, all the new machines thus far described are of what is