In a practical lamp we must have not only a pair of carbons for producing the arc, but also means for supporting these carbons, together with suitable arrangements for leading the current to them and for maintaining them at the proper distance apart. The carbons are kept separated the proper distance by the operating mechanisms which must perform the following functions:

Fig. 33. Distribution Curve for

Fig. 33. Distribution Curve for.

D. C. Arc Lamp (Vertical Plane).

1. The carbons must be in contact, or be brought into contact, to start the arc when the current first flows.

2. They must be separated at the right distance to form a proper arc immediately afterward.

Fig. 34. Distribution Curve for A. C. Arc Lamp (Vertical Plane).

Fig. 34. Distribution Curve for A. C. Arc Lamp (Vertical Plane).

3. The carbons must be fed to the arc as they are consumed.

4. The circuit should be open or closed when the carbons are entirely consumed, depending on the method of power distribution.

The feeding of the carbons may be done by hand, as is the case in some stereopticons using an arc, but for ordinary illumination the striking and maintaining of the arc must be automatic. It is made so in all cases by means of solenoids acting against the force of gravity or against springs. There are an endless number of such mechanisms, but a few only will be described here. They may be roughly divided into three classes:

1. Shunt mechanisms.

2. Series mechanisms.

3. Differential mechanisms.