Tesla's Alternating Current Meter

A novel meter for alternating currents, designed by Nikola Tesla, is illustrated in the accompanying diagram. The invention is based upon the fact that when a high tension discharge is made to pass from a conductor through rarefied gas, minute particles are thrown off from the conductor, the amount of detachment depending upon the strength of the current. In a suitable tube of glass are sealed platinum leading wires H coated inside the tube with insulating material. Sticks of carbon A, B are connected to these platinum terminals and the tube exhausted and sealed. The two carbon sticks A, B are electrically connected with the secondary of an induction coil, the primary of which is in series with the alternating circuit in which the current consumption is to be measured. For example, F represents a group of incandescent lamps, and E the primary of the induction coil. The difference of potential at the terminals of the secondary is proportionate to the primary current, and, therefore, to the current used by the lamps. The discharge from one conductor, A, to the other, as B, produces a detachment of infinitesimal particles along the entire length of the sticks, and after a period of service the latter increases in resistance. The amount of detachment is said to be proportional to the difference of potential between the two conductors. Hence, the increase of their resistance is in a definite proportion to the number of lamp hours. By measurements taken at the start and finish of any period of service the amount of energy used during such period of service may be computed. The action may be confined to confronting faces of the carbon sticks by coating the other faces with insulating material.