TRANSACTIONS OF SECTION A. 571 



" charging coil," all connections being as short as possible, so as to reduce, as much 

 as it is practicable, both the resistance and self-induction of this circuit. 



On the front side of the box containing the condenser there are mounted two 

 binding posts for connection with the lines, two small fuses, and a reversing switch. 

 In addition two adjusting screws are provided for raising and lowering the iron 

 core within the charging coil as a convenient means for varying within consider- 

 able limits the current of supply, and regulating thereby the discharge of the 

 secondary circuit. In adjusting the core the left-hand screw should be unscrewed 

 tirst, as it performs merely the function of a check-nut. 



The mode of operation may be explained in current language as follows :— 



At the start the spring contacts being closed and the condenser practically 

 short-circuited, a strong current passes through the " charging coil," attracting the 

 armature fastened to the lower spring, and separating the contacts. Upon this the 

 energy stored in this coil, assuming the form of a high-tension discharge, rushes 

 into the condenser, charging the same to a high potential. The current through 

 the coil now subsiding, the attraction exerted upon the armature ceases, the spring 

 reasserts itself and again closes the contacts. With the closing of the latter the 

 condenser is discharged through the primary, and simultaneously a strong current 

 from the source of supply again rushes through the charging coil, and energy is 

 stored for the next charge of the condenser, this process being repeated as often as 

 the spring opens and closes the contacts. 



By means of this simple arrangement certain advantages over ordinary coils 

 are secured, the chief being the absence of tine wire in the secondary, the quality 

 of the effects produced, and efficiency. 



The photograph, showing the instrument in action with two loops of cotton- 

 covered wire attached to the discharge rods, conveys an idea of the pressures 

 obtained. The outer wire loop was in the experiment only 22 inches in diameter, 

 to enable its being properly shown in the print, but it could have been much 

 larger, since two parallel wires, 15 feet long, may be stretched from the secondary 

 terminals of the instrument, and practically the entire space between them, 3^ 

 inches wide, is seen in the dark covered with fine streamers — that is, a surface of 

 over 4 square feet — and yet the energy taken from the supply circuit during the 

 performance is less than 35 watts. It is practicable, by the use of the principle 

 described, to obtain sparks of 1 foot in length with an expenditure of energy of 

 less than 10 watts. 



3. An Electric Curve Tracer. By Professor E. B. Rosa. 



One of the most interesting and fruitful methods of investigation of alternate 

 current phenomena is the tracing of the forms and phases of current and electro- 

 motive force waves. But the practicability of this method of investigation and 

 testing has been seriously limited by the great labour of obtaining the curves, and 

 the insufficient accuracy of the curves when obtained. Although various methods 

 are employed for determining the quantities from which to plot the waves of 

 current and electromotive force, yet in nearly every case an instantaneous coutact- 

 maker is used, and the contact brush is advanced by hand, step by step, settings 

 being made on a graduated cii-cle. Headings are taken on a voltmeter, electro- 

 meter, or galvanometer, and subsequently points are plotted out on cross-section 

 paper, and a smooth curve drawn through them. Because of the great labour 

 involved, comparatively few points are usually found, and hence the curves are 

 only approximately determined. To reduce the labour and increase the speed of 

 working would enable a greater number of points to be determined, and so give 

 more faithful representations of current curves. This could be accomplished by 

 some arrangement that would do the most laborious part of the work mechani- 

 cally, and, if possible, automatically. Hence, if successive settings of the brush of 

 the contact-maker could be made quickly and easily, and the curve printed out 

 automatically, so as to eliminate the necessity of taking readings and plotting the 

 points, the thing would be done. It was to accomplish this end that I had an 



