Everything will occur, then, as if we caused the ten-bobbin solenoid to move instead of the brushes. This granted, and the brushes being in any position whatever, let us send a current into the apparatus, and place therein a soft iron cylinder. By virtue of a well known law, such cylinder will remain suspended in the interior of the solenoid, and its longitudinal center will place itself at so much the greater distance from that of the solenoid the more the current increases in intensity. It would even fall entirely if the current had not an intensity above a minimum value dependent upon many elements concerning which we have not now to occupy ourselves. We will suppose the current intense enough to keep the distance of the two centers much below that which would bring about a fall of the cylinder. When such a condition is fulfilled, it is found that if we try to remove the iron cylinder from the equilibrium that it is in, we must apply a pressure that increases with the amount of separation, just exactly as if it were suspended from a spring. It results from this fact that if we displace the brushes a distance equal to the thickness of one plate of the collector, the active solenoid will undergo the same displacement, and its longitudinal center will move away from that of the iron cylinder, and that the attraction exerted upon the latter will increase. It will not be able to assume its first value, and equilibrium cannot be re-established unless the cylinder undergoes a displacement identical with that of the solenoid. Now, as this latter depends upon the motion communicated to the system of brushes, we see that, definitively, the cylinder will faithfully reproduce the motion communicated to the brushes by the hand of the operator. This apparatus, then, constitutes a genuine electric servo-motor in which the current is never interrupted nor modified in quantity or direction, no more indeed than the magnetization developed in the soft iron cylinder. Everything takes place as if the iron cylinder were suspended in a solenoid ten centimeters in length that was caused to rise and fall; with the difference that the weight of the cylinder exerts no action on the hand of the operator. [Illustration: ELECTRIC POWER HAMMER.]