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.

Fig. 36. Differential Mechanism for D. C. Arc Lamp.

The methods of supporting the carbons and feeding them to the arc may be divided into two classes: 1. Red-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 arrangement 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 advantage 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 lumps 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.

Fig. 37. Rod-Feed Mechanism.