(approximately 4) divided by 0.9, a total of 143, or approximately 286 volts across the anode terminals of the bulb.

A reactance is placed in the direct-current circuit to give the necessary bridging over the zero point. This coil should have sufficient capacity to maintain the direct-current flow during the period in which the alternating-voltage wave is less than that across the terminals of the storage battery. To maintain the current flow at a constant value would require a reactance of infinite proportion. Therefore, to obtain a minimum size, the current supplied is allowed to pulsate a certain amount. For the ordinary battery installation the smoothing out need only be sufficient to prevent the bulb going out. For some special purposes, however, such as batteries used in telephone exchanges, where they will be charged while supplying current to talking circuits, it is ncessary to use a reactance of considerable proportions in order that no noise be produced in the telephone circuits.

It is possible to design the auto-transformer supplying the bulb so as to obtain the necessary reactance for sustaining the direct-current circuit without the use of the additional reactance. This result may be obtained without in any way impairing the regulation or lowering the power-factor of the alternating-current supply circuit. In order to give a proper control of the charging current, it is necessary to use a resistance in series when charging from a constant-potential supply. This resistance may be placed in the alternating-current side of the rectifier and, on the score. of efficiency, a reactance should be used, thus saving the energy that would be lost if a resistance were used in the battery circuit. In order that this reactance value may not be unnecessarily large, especially when delivering direct-current voltage below the rated maximum of the outfit, taps are provided on the auto-transformer which permit of using a reactance value just sufficient to give the necessary stability to the charging current.

To put the outfit in operation it is necessary to break down the electrode resistance of the cathode. This is accomplished by a small arc produced between the mercury forming the cathode and that in a small auxiliary electrode. The bulb is tilted so that the mercury in the two electrodes makes contact, thus closing a local circuit which at the break produces the small arc necessary for starting.

The efficiency of an outfit of this character is high compared to that of a small motor-generator set. It is determined mostly by the voltage of the direct-current circuit, as the voltage loss in the bulb is practically constant irrespective of the total voltage of the circuit or the current delivered. As stated before, the loss in the bulb amounts to about 15 volts; therefore, to obtain 110 volts direct current, the voltage of the transformer should correspond to a direct-current voltage of 125, the efficiency of the bulb thus would be or 88 per cent. With an 125

efficiency of the transforming apparatus of 90, the net efficiency of conversion from alternating current to direct current would be over 79 per cent. The efficiency of a small motor-generator set. would probably be 60 to 65 per cent.

The power-factor of the outfit, when used without the adjusting reactance in the alternating-current circuit, would be about 90 per cent. When used with this reactance it will, of course, be lower. It should be possible, however, to operate with a power-factor of 80 per cent.

From the standpoint of the central-station man, the most important development of the rectifier is that for the operation of constant-current, direct-current arc lamps. Within the last few years there has been a very great increase in the use of series alternating-current arc lamps for street lighting. This has in a large measure been due to the fact that they can be operated from the existing alternating-current lighting generators with simple and efficient transforming apparatus. With the advent of the new high-efficiency, direct-current, series arc lamp, it would seem like a step backward to revert to the old constant-current gen

erator.

For the supply of constant-current circuits, it is necessary that the function of a regulator be combined with that of a rectifier, and the obvious method, therefore, is to use a regulator of the same type as that employed in connection with alternatingcurrent lamps and convert the current delivered by this regulator into a direct current. With the exception that the transformer supplying the alternating-current to the rectifier is of the constant-current regulating type, the arrangement and function of each part is similar to that of the outfits supplying constant po

tential. However, the difference in the characteristics of the load require a corresponding modification in each detail part of the system. A description of an outfit designed primarily for the operation of the new high-efficiency arc lamps will illustrate the general features of this class.

The regulating transformer is in general similar to that used for alternating-current lamps. The secondary, however, is wound for the higher voltage required on the rectifier terminals. a middle tap being furnished for the negative terminal of the direct-current circuit, and in addition to this some extra taps are provided near the ends of the secondary winding, to provide for operation on loads below the rated capacity and at a power-factor corresponding to that obtained on a full load.

An auxiliary secondary winding consisting of a few turns wound directly in the transformer core is used for operating the starting device of the bulb.

The terminals of the transformer secondary are connected direct to the positive terminals of the bulb.

From the negative terminal of the bulb, connection is made to the reactance, the remaining terminal of this reactance forming the positive supply terminal for the load.

For starting, two methods have been employed. In one of these methods, two additional electrodes are used in the bulbs. These auxiliary electrodes are connected to a small transformer which is fed from the auxiliary secondary on the main regulating transformer. A connection is made from the negative electrode of the bulb to the middle point of this small transformer, the whole thus constituting a complete rectifying outfit on a smaller scale. The auxiliary electrodes are so placed that by tilting the bulb the mercury forming the cathode will bridge over and short-circuit them, the spark that follows the breaking of this short-circuit serving to start the operation of the rectifier. Should the lamps break their arcs, due to a momentary drop of the supply voltage, they would immediately restart, as the auxiliary local circuit would keep the cathode of the bulb broken down or alive.

The local rectifying circuit is so designed as to consume a comparatively small amount of energy, the terminal voltage of its transformer being about 150 volts, which is mostly con

sumed in inductive resistance, the current flow being approximately 0.5 ampere. This arrangement, while somewhat complicated, has the advantage that the lamps are not likely to go out on a fluctuating primary voltage and is therefore adapted for lamps operated from low-frequency railway or power circuits. When operated from lighting circuits of average regulation the following arrangement is to be preferred: In this, the auxiliary positive electrodes are omitted and two negative electrodes are used. These electrodes are connected to the terminals of a small reactance coil and the positive terminal of the direct-current circuit is connected to the middle point of the winding of this coil. An alternating voltage of about 20 volts is impressed across the terminals of this coil. Thus, by tilting the bulb so that the mercury of the two negative electrodes will bridge across, a spark will be formed which will start the rectifier in operation. The direct current passes through one half or the other of the small reactance, depending upon which electrode happens to be started, this being determined by the direction of the current wave that formed the starting spark. When the rectifier is in operation there is no current flowing through the bulb other than that delivered to the outside circuit. The heating effect, therefore, is reduced to a minimum.

The bulb is mounted in a wooden cage and placed in the tank that contains the regulating transformer and auxiliary apparatus. The external appearance of the complete device is similar to that of the oil-cooled regulating transformer used for series alternating-current lamps.

Connections are made from the bulb to contacts on bottom of the wooden cage, which contacts in turn register with a set attached to the framework of the regulator. When the cage is lifted out of the oil for inspection or renewal of bulb all connections are automatically broken, and when cage is replaced the connections are automatically made.

The arrangement of placing all the parts together in a single tank has a number of advantages, among which may be mentioned: There is no exposed high-tension wiring to collect dust and through leakage affect the operation of the rectifier bulb, it being found by experience that a very slight leakage from the connecting wires between the rectifier bulb, the reactance and the

transformer very greatly increased the tendency of the bulb to short-circuit or go out. The bulb is protected from any chance of accidental injury.

The life of the bulb is very greatly increased. This latter effect is due principally to the cooling effect of the oil and is a very vital point. Tests that have been made show that the average life of a bulb that is operated in air is about 700 hours. When the bulb is operated in oil there is apparently no depreciation whatever, the life depending somewhat on accidental circumstances and averaging several thousand hours.

As to the general operation and efficiency of the rectifier outfit, the results obtained are far superior to that of a constantcurrent generator. The percentage of loss in the bulb is practically zero, amounting to less than 1 per cent in an outfit supplying 50 arc lamps. The losses in the regulating transformer and reactance will total about 8 per cent more, which will give a total efficiency of conversion from alternating to direct current of above 90 per cent.

The efficiency of the best arc machine in the large sizes will be less than 80 per cent, and if we take 90 as the efficiency of the motor driving this generator and 97.5 per cent as that of the step-down transformer supplying the motor, we have a total efficiency alternating to direct current of slightly over 70 per

cent.

The only respect in which the rectifier outfit will suffer in comparison with the arc machine is that of power-factor. Due to the steadying reactance required in the regulating transformer, the power-factor corresponds to that of the alternating-current series arc system and is somewhat in excess of 70 per cent.

THE PRESIDENT: If there is no discussion on this paper, we will proceed to one of the papers on the programme for to-morrow morning, Recent Developments in Protective Apparatus, by Mr. D. B. Rushmore, of Schenectady.

[Mr. Rushmore not being present, the paper was read by Mr. Delafield Dubois, to whom the author requests that acknowledgment be made for valuable assistance in preparing the paper.-EDITOR.]