262 PROCEEDINGS OF THE AMERICAN ACADEMY. 



shape of the pole pieces the field between them is variable, being 

 denser at the points of the stars, so that when the field moves past 

 the armature an E. M. F. will be set up in the latter. 



One disadvantage of this design is that the E. M. F. is small, 

 owing to the very large non-effective field ; for it is only the variation 

 of the field that is of service in producing the E. M. F. By properly 

 shaping the pole pieces a curve of E. M. F. was obtained, which, 

 when plotted, was found to be nearly sinusoidal in form. 



The second machine was constructed by Messrs. C. L. Norton 

 and P. H. Thomas, of the class of 1893, and forms the subject of the 

 present paper. The special form of dynamo used was designed in 

 accordance with a suggestion from them. The frequency is some- 

 what low, being 400 complete alternations per second. When 

 another machine is built, it will be so designed that the superior 

 limit will be raised to 2,000 complete periods per second. 



Both the machine of 1890 and that of 1892 embody the same 

 general features. Each has two independent armatures, one fixed, the 

 other movable in phase relation to the first. This relation is capa- 

 ble of measurement. At the outset it was determined that in these 

 machines the armatures should contain no iron, and that they should 

 remain stationary, in order to eliminate any possible microphonic 

 action at the brushes. 



The machine, which is shown in place ready for use in Figure 1 ? 

 will now be described. It consists, in brief, of two dynamos, with 

 revolving fields mounted on one shaft to insure definite phase rela- 

 tion. The armature of one is fixed, while that of the other may be 

 given any desired angular advance or retardation in reference to the 

 first by means of a tangent screw. This advance can be read off 

 from the graduated head of the screw. The fields are independent 

 of one another. In addition to the dynamos the shaft carries a con- 

 tact arrangement for mapping wave-forms, and a stroboscopic disk for 

 determining the speed of rotation. 



One of the field magnets is shown in plan in Figure 3 and in 

 section in Figure 2. It consists of two star-shaped pieces of cast 

 iron, keyed to the sh ft and having hubs of such a length that tho 

 faces of the teeth are -fa" apart, thus leaving a gap in which is the 

 armature, \" thick. The castings are recessed to receive the field coils, 

 which are about one square inch in section and contain 250 turns of 

 No. 16 B. & S. double cotton-covered wire. The coils were wound on 

 forms, paraffined and taped, and were inserted when the machine was 

 assembled. The terminals are brought out through bushed holes and 



