Such lamps have the disad vantage of a high resistance at the start-450 ohms or more— and are difficult to start on series circuits, due to the high voltage required. They tend to maintain a constant voltage at the arc, but do not aid the dynamo in its regulation, so that the arcs are liable to be a little unsteady.
Series Mechanisms. With the series-lamp mechanism, the carbons are together when the lamp is first started and the current, flowing in the series coil, separates the electrodes, striking the arc. When the arc is too long, the resistance is increased and the current lowered so that the pull of the solenoid is weakened and the carbons feed together. This type of lamp can be used only on constant potential systems.
Fig. 35 shows a diagram of the connection of such a lamp. This diagram is illustrative of the connection of one of the lamps manu factured by the Western Electric Company, for use on a direct-current, constant-potential system. The symbols + and — refer to the termi nals of the lamp, and the lamp must be so connected that the current flows from the top carbon to bottom one. R is a series resistance, adjustable for different voltages by means of the shunt G. F and D are the controlling solenoids connected in series with the arc. B and C are the positive and negative carbons respectively, while A is the switch for turning the current on and off. II is the plunger of the solenoids and I the carbon clutch, this being what is known as a carbon-feed lamp. The carbons are together when A is first closed, the current is excessive, and the plunger is drawn up into the so lenoids, lifting the carbon B until the resistance of the arc lowers the current to such a value that the pull of the solenoid just counter balances the weight of the plunger and carbon. G must be so adjusted that this point is reached when the arc is at its normal length.
Differential Mechanisms. In the differential lamp, the series and shunt mechanisms are combined, the carbons being together at the start, and the series coil arranged so as to separate them while the shunt coil is connected across the arc, as before, to prevent the carbons from being drawn too far apart.
This lamp operates only over a low-current range, but it tends to aid the generator in its regulation.
Fig. 36 shows a lamp having a differential control, this also being the diagram of a Western Electric Company arc lamp for a direct current, constant-potential system. Here S represents the shunt coil and M the series coil, the armature of the two magnets A and A' being attached to a bell-crank, pivoted at B, and attached to the carbon clutch C. The pull of coil S tends to lower the carbon while that of M raises the carbon, and the two are so adjusted that equilibrium is reached when the arc is of the proper length. All of the lamps are fitted with an air dashpot, or some damping device, to prevent too rapid movements of the working parts.
The methods of supporting the carbons and feeding them to the arc may be divided into two classes: 1. Rod-feed mechanism.
2. Carbon-feed mechanism.
Rod-Feed Mechanism. Lamps using a rod feed have the upper carbons supported by a conducting rod, and the regulating mechanism acts on this rod, the current being fed to the rod by means of a sliding contact. Fig. 37 shows the ar rangement of this type of feed. The rod is shown at R, the sliding contact at B, and the carbon is attached to the rod at C.
These lamps have the ad vantage that carbons, which do not have a uniform cross section or smooth exterior, may be used, but they possess the disadvantage of being very long in order to accommodate the rod. The rod must also be kept clean so as to make a good contact with the brush.
Carbon-Feed Mechanism. In carbon-feed lamps the controlling mechanism acts on the carbons directly through some form of clutch such as is shown at C in Fig. 38. This clamp grips the carbon when it is lifted, but allows the carbon to slip through it when the tension is released. For this type of feed the carbon must be straight and have a uniform cross-section as well as a smooth exterior. The current may be led to the carbon by means of a flexible lead and a short carbon holder.