840 KLECTRICITY DURING THE NINETEENTH CENTURY. 



wire through the center of the iron ring, as in the Gmmme construc- 

 tion. The several coils of the drum were still connected, as in 

 Gramme's machine, to the successive strips of the commutator. 



In modern dynamos and motors the armature, usually constructed 

 of sheet-iron punchings, is a ring with projections, as in Paccinotti's 

 machine, and the coils of wire are in most cases wound separately and 

 then placed in the spaces between the projections, constituting in fact 

 a form of drum winding. In the early seventies a few Gramme ring 

 and Siemens drum machines had been applied to the running of arc 

 lights, one machine for each light. There were also some Gramme 

 machines in use for electroplating. 



In all dynamos in practical use the currents which are generated are 

 alternating currents, as they are called. Such currents are character- 

 ized ])y rapid changes of direction or reversals. These occur man}'^ 

 times per second, and when such currents are to be made into contin- 

 uous currents flowing in one direction, the machine is provided with a 

 conmuitator for connecting the coils to the circuit, so that the current 

 will always flow in it in the same direction. Great numbers of dynamos, 

 however, are used without commutators for changing the direction of 

 their currents. In such machines the circuit receives, instead of con- 

 tinuous currents, waves of current or alternating currents. As with 

 sound, the waves have a pitch — i. e., the}^ follow each other at a certain 

 number of times per second. In usual practice there will be from 25 

 waves, which would be a low period, up to 150 or more per second, but 

 machines can be constructed to produce alternating currents of many 

 thousand waves or cycles (as they are termed) per second for special 

 uses. When an alternating current flows from a line, there are times 

 when the current is changing from one direction to the other and 

 when there is actually no current, and these are called the zeros or 

 dead points of the current. Much of the machinery developed in later 

 years has been of the alternating-current type, generators, motors, etc. , 

 utilizing these rapidly reversing currents. 



At the Centennial Exhibition, held at Philadelphia in 1876, but two 

 exhibits of electric-lighting apparatus were to be found. Of these one 

 was the Gramme and the other the Wallace-Farmer exhibit. The 

 Wallace-Farmer dynamo machine is a type now obsolete. It was not a 

 good design, but the Wallace exhibit contained other examples reflect- 

 ing great credit upon this American pioneer in dynamo work. Some 

 of these machines were very similar in construction to later forms 

 which went into very extensive use. The large searchlights occasion- 

 ally used in night illumination during the exhibition were operated by 

 the current from Wallace -Farmer machines. The Gramme exhibit 

 was a remarkable exhibit for its time. Though not extensive, it was 

 most instructive. There were found in it a dynamo running an arc 

 lamp, a large machine for electrolytic work, such as electroplating and 



