Closely allied to the practical use of the incandescent lamp is the method of supplying and regulating the current from the dynamo. Although the alternating current is used for arc light, only the continuous current can be used for the incandescent lights, and the relation of the dynamo and the incandescent lamps is shown in Fig. 53, in which L represents the lamps between the main conducting wires leading from the dynamo, which latter has the coils of the field magnets arranged in a shunt or branch circuit, in which is interposed a regulator R in the form of a resistance coil with movable switch lever, by which more or less of the current is allowed to flow through the field magnet coils to suit the work being done. In late years automatic regulators have been provided for accomplishing this result. In Fig. 54 is shown what is known as the Edison “three wire system,” patented March 20, 1883, No. 274,290. In this two dynamos are used as at D¹ D², and the three wires emerge from the dynamos, one from the negative pole of one dynamo, another from the positive pole of the other dynamo, and the third or middle one is connected to both the other poles (positive and negative), of the two dynamos. For purposes of illustration, this may be compared to a three-storied arrangement of current, the upper wire representing the third story, the middle wire the second story, and the bottom one the first story. The fall from either story to the next represents the working energy, but from the top wire to the bottom would be equal to a fall from the third story to the first. The purpose of this arrangement is to save expense in copper wire, for while three main wires are used instead of two, the aggregate weight of the wires (when the lamps are arranged as shown), may be made so much less than two heavy wires as to make a very great saving in copper. The uses of the incandescent light are legion. Besides those which are of common observation it is used for lighting the interior of mines, caves, and the dark apartments of ships, and does not foul the air. It is also used by divers in submarine operations; in the formation of advertising signs, and in pyrotechnics, but perhaps one of the most extraordinary uses to which it has been put is in exploring the interior of the human stomach and other cavities of the body, a patent for which was granted to M. C. F. Nitze, No. 218,055, July 29, 1879. When an electric lamp is arranged with the opposite ends of the carbon burner connected, one to the outgoing, the other to the incoming wires from a dynamo, so as to be bridged across, this arrangement is said to be “in multiple” or “in parallel,” and the lamps bear the analogy of horses drawing abreast, and when the opposite ends of the carbon burner are placed in a gap or break in either the outgoing or the incoming wire, the arrangement is said to be “in series,” and the lamps bear the analogy of horses in tandem.