Figure 61 illustrates the Pilsen lamp, in which the positive Carbon G runs on rollers rr through the hollow interior of two solenoids or coils of wire MM' and carries at its middle a spindle-shaped piece of soft iron C. The current flows through the solenoid M on its way to the arc, but a branch or shunted portion of it flows through the solenoid M', and as both of these solenoids act as electromagnets on the soft iron C, each tending to suck it into its interior, the iron rests between them when their powers are balanced. When, however, the arc grows too wide, and the current therefore becomes too weak, the shunt solenoid M' gains a purchase over the main solenoid M, and, pulling the iron core towards it, feeds the positive carbon to the arc. In this way the balance of the solenoids is readjusted, the current regains its normal strength, the arc its proper width, and the light its brilliancy. Figure 62 is a diagrammatic representation of the Brush arc lamp. X and Y are the line terminals connecting the lamp in circuit. On the one hand, the current splits and passes around the hollow spools H H', thence to the rod N through the carbon K, the arc, the carbon K', and thence through the lamp frame to Y. On the other hand, it runs in a resistance fine-wire coil around the magnet T, thence to Y. The operation of the lamp is as follows: K and K' being in contact, a strong current starts through the lamp energising H and H', which suck in their core pieces N and S, lifting C, and by it the "washer-clutch" W and the rod N and carbon K, establishing the arc. K is lifted until the increasing resistance of the lengthening arc weakens the current in H H' and a balance is established. As the carbons burn away, C gradually lowers until a stop under W holds it horizontal and allows N to drop through W, and the lamp starts anew. If for any reason the resistance of the lamp becomes too great, or the circuit is broken, the increased current through T draws up its armature, closing the contacts M, thus short-circuiting the lamp through a thick, heavy wire coil on T, which then keeps M closed, and prevents the dead lamp from interfering with the others on its line. Numerous modifications of this lamp are in very general use. Davy also found that a continuous wire or stick of carbon could be made white-hot by sending a sufficient current through it, and this fact is the basis of the incandescent lamp now so common in our homes.