172 On Measuring Electric Currents of great Strength. 



there are four of these, each about 10" long, 3f" deep, and 

 22" wide. The armature resistance is 0*129 ohm, the field 

 resistance 0*212 ohm, thus making *341 ohm for the total 

 internal resistance. The total weight of wire in the machine 

 is 483 pounds ; or the weight of wire is nearly 18 per cent, of 

 the total weight of the machine. 



Siemens Machine, or Heffner-von-Altenek Machine, built 

 by the Siemens Brothers. — This machine is 61" in length, 28" 

 in breadth, and 12" in height. The armature is nearly 34" 

 long and 9 \ ff external diameter. It is formed by winding 98 

 pounds of two insulated wires longitudinally, and in eight di- 

 visions, around a thin and hollow brass cylinder. Within this 

 hollow cylinder is a hollow stationary cylinder of cast iron, 

 supported by bearings that pass through the brass cylinder. 

 The commutator has eight divisions, which are eight sector- 

 shaped sheets of brass, insulated from but attached to the face 

 of a plate which is outside of one of the bearings of the brass 

 cylinder. Two collectors or brushes trail upon and press 

 against these sectors : these brushes have a bearing so exten- 

 sive as to short-circuit or bridge over the edge of two sectors. 

 The spark of the commutator is quite insignificant. This ma- 

 chine differs from all others in this respect — the armature 

 simply moves a wire through a field of force, and not a soft- 

 iron core covered with wire. The resistance of the entire 

 circuit, field of force, corrected for conductivity, is *586 ohm. 

 The normal velocity of the machine is from 370 to 380 revolu- 

 tions per minute. 



Wilde Machine. — This resembles in some respects the Hjorth 

 machine of 1855 with the permanent magnet omitted. It has 

 two armature circuits — one with current uniform in direction 

 for the purpose of maintaining the magnetism of the field, and 

 the other for producing the electric light. The current from 

 this last circuit is a to-and-fro current, without commutator. 



" The armature wire weighs 28 pounds, and is divided into two 

 circuits : about 7 pounds of it, having a resistance of *454 ohm, 

 furnishes the current which maintains the field. The remainder 

 (21 pounds), having a resistance of *074 ohm, maintains the 

 to-and-fro current. About 325 pounds of wire are distributed 

 in 24 coils to make up the electromagnetic field, which has a 

 resistance of 2 '83 ohms. These coils are 10 J" in length and 

 3J-" in external diameter, having soft round cores 2" in dia- 

 meter. There are 24 armature cores and coils, one half on 

 each side of a central cast-iron wheel 1J" thick. The central 

 diameter of this wheel is 18" nearly. The whole weight of 

 wire in this machine is nearly 354 pounds." The normal 

 velocity of the machine is about 600 revolutions. A greater 



