574 REPORT— 1897. 



and one wishea a faithful representation of the curve, two teeth are taken at a 

 time, and 180 points plotted in one single wave-length, which extends half-wave 

 around the recording cylinder. 



If the generator is a two-pole machine, four teeth will be taken at one step on 

 the contact-maker and two on the recording cylinder. If it is a four-pole machine, 

 an equal number of teeth will he taken on each. If it is an eight-pole machine, 

 one tooth is taken on the contact-maker and two on the recording cylinder, and 

 so on. In each case the resulting curves will he drawn to the same horizontal 

 scale. 



Special attachments are provided for printing copies of original curves, for 

 making curves of magnetisation and hysterisis curves, and also for taking power 

 curves — in the latter case not by multiplying corresponding ordinates of current 

 and electromotive force curves, but plotting directly from the settings of the 

 contact piece P. 



The accuracy of the work of the instrument is illustrated by curves, representing 

 the current flowing into a condenser from the secondary of a transformer (which 

 were exhibited). The capacity of the condenser for the second curve is double 

 that of the first, other circumstances being the same. The curves show a little 

 more than a wave length and a half of the fundamental, but the character of the 

 curves is determined by their upper harmonics, the natural period of the condenser- 

 transformer circuit being such as to amplify very small upper harmonics in the 

 electromotive force of the dynamo, which had a toothed armature. In the second 

 curve the period is larger (because the capacity is greater), and the free vibrations, 

 which are superposed on the fundamental, are accordingly fewer in number. 

 Comparing one half-wave with another, it is evident that the curve-tracer has 

 done its work faithfully. Without a large number of points in the space of one 

 wave we should fail to apprehend the true character of such curves. The instru- 

 ment lends itself to a great variety of purposes. One can study the actions and 

 reactions in dynamos and motors, of single and polyphase varieties ; in transformers 

 of all types, and of special devices in practical or abstract research. By means 

 of a two-part commutator on the shaft of the contact-maker the oscillating cur- 

 rents of condenser charges and discharges can be delineated, and the period 

 measured. Curves showing the rise and fall of current in inductive circuits when 

 the current is made and broken can be drawn, and the self-induction thereby 

 measured. 



I wish to express my obligations to Mr. 0. S. Blakeslee, the accomplished 

 mechanician of the college, for his assistance in designing the mechanical features 

 of the Curve Tracer, and for his skill in constructing the instrument. 



The equations of the two curves are as follows : — 



1st. I = 8-79 sin {x - 18°50') - 1-02 sin (3.r - 44°18') + 2-55 sin (6.r + 84°31') 

 - -41 sin7.r - 2-95sin (9a- - 5°43') + l-88sin (11a- + S4°36') + 8-OSsin 

 (13.1- + 10°70 + 5-45 sin (15.r - 59°56'). 



2nd. I = 18-75 sin (.r - 21°60 - 2-18 sin r3.c - 70°1') - 6-86 sin {5x + 61 °48") 



- 1-56 sin (7.1- - 84°70 + 5-30 sin (9.r + 66°14') + 098 sin (ll.c - 83°30') 

 + 4-15 sin (13,r - 43°300 + 3-59 sin (15.r - 86°30'). 



The 17th and higher harmonics not present to an appreciable extent. 



4. On the Use of the Interferometer in the Study of Electric Waves. 

 By G. F. Hull, University of Chicago. 



An interferometer for electric waves, constructed after Michelson's form, -was 

 used to analyse electric radiation. A Branly receiver (small nails in oil) and 

 different forms of Righi's vibrators were used. The following conclusions were 

 arrived at : — 



1. The interference curve depends on both vibrator and receiver, and the 

 influence of each of these varies. 



