pencils, and the greatest distance the current would jump on suddenly breaking the circuit was determined. This distance corresponded to a little less than 8,000 volts, showing that the electro-motive force had risen between 1,500 and 2,000 volts. This can be accounted for by the fact that it takes an appreciable moment of time for the arc on the commutator to be carried from one brush to the other. Owing to each segment extending about sixty degrees around the commutator, the time required to short circuit the commutator is just one-third of what it would be in the closed coil machine; consequently, I think the statement that in a closed coil machine the voltage would rise to fully double its normal value, under the same conditions, would be a very conservative one; in fact, I have many reasons for believing that it would go considerably beyond this, provided, of course, that the insulation of the commutator did not break down. If this occurred, the machine would necessarily be unfit to run again until repaired. This peculiarity, if I may call it such, of the open coil machine is especially important where underground circuits are employed, since a breakdown in the insulation due to an abnormal rise in voltage is something that can only be repaired at considerable expense. Knowing that underground circuits would be of interest to central station engineers, I arranged, through the courtesy of Mr. E. A. Leslie, of the Manhattan Electric Light Company, of New York, to make some determinations asto the rise in voltage when a long underground circuit was suddenly broken. An open coil 125-light machine was connected to an eighteen-mile underground circuit supplying current to 125 arc lamps. The voltmeter registered 6,850 volts at the machine terminals. The circuit was then broken a number of times, with the carbon points in shunt with the break. At first, the carbons were some tests on VOLTS separated by a distance corresponding to 10,000 volts, and this distance was then gradually reduced until the 7000 6832 6500 6000 5500 6000 4500 4000 3500 Fig XI current jumped the air space. This indicated only The carbons were then set for 6,000 5,000 volts. volts, with the result that the current jumped twice in breaking the circuit eight times, showing that we had about reached the limit. It was then decided to try breaking the circuit near the center. Under these conditions, 5,000 volts was the highest reading we obtained, but there was a marked difference in the time of discharge. In the first case, there were two discharges in rapid succession within half a second 100 Volts. after the break; while in the second case, fully two seconds elapsed before the current jumped. In both cases, there were two discharges before the arc was established, the first one being short and simply resembling a spark. From this it will be seen that the capacity of the circuit more than counterbalances any rise in voltage due to the self-induction of the machine and line. With these results before us, we can safely assume that the insulation of a cable will not be overstrained by an accidental break in the line. All these considerations led the company with which I have the honor to be connected to adhere to this, the open coil, type of machine in designing their large units. Fig. XI is a characteristic curve of this machine without any regulator. The readings were all taken Fig. XIII 6000 VOLTS at the sparkless position of commutation. This is remarkable from the fact that after we get over the bend the curve is almost perpendicular, and is probably the nearest approach to a constant current machine ever attained. By winding more wire on the armature, we could have made this machine deliver a constant current of 9.6 amperes at all loads, without shunting any of the current from the |