572 



REPORT — 1897. 



instrument constructed last year in our college machine shop. The instrument 

 worked well and gave some very elegant results. But there appeared in its use 

 several practical defects which experience showed how to remedy, and this year I 

 have constructed a new instrument, which may be called an Electric Curve 

 Tracer, and which will now be briefly described. 



The instrument consists of three parts: (1) the Contact-Maker, (2) the 

 Measuring Selenoid, and (3) the Recording Cylinder. The Contact-Maker is 

 joined by a rod and flexible couplings to the shaft of the dynamo, which produces 

 the current to be delineated, or to a synchronous motor which is driven by that 

 current. Hence the shaft of the contact-maker, and with it a hard rubber disc 

 six inches in diameter, revolves with the speed of the armature of the alternator. 

 The brush which, once in every revolution, makes contact with a knife-edge let; 

 into the edge of the hard rubber disc, is carried by an arm which is advanced step 

 by step by a ratchet wheel and gearing. The pawl of the ratchet is actuated by 

 an electro-magnet, and the step of the brush is half of one degree for each tooth 

 of the ratchet wheel. Any number of teeth from one to six may be taken at each 

 step, according to the position of the stop. The current through the electro- 

 magnet, which advances the brush, is made by the operator at the measuring and 

 recording apparatus, which may be at a distance. 



The Measuring Solenoid consists of a single layer of insulated wire wound 



Fig. 1. 



upon a hard rubber rod 80 centimeters in length ; along one element of this 

 solenoid the insulation is removed. A current from two or three storage cells 

 passes through the solenoid from A to B, and by means of a voltmeter and 

 rheostat the difference of potential between A and B is kept constant. This 

 spiral has so much greater length of wire than a single wire, such as that on a 

 slide wire bridge, that it can maintain a much greater difference of potential, and 

 serves the purpose better than a single wire would do. Let the current to be 

 delineated pass through the non-inductive resistance C D. We measure the instan- 

 taneous difference of potential at the terminals of C D by matching it against the 

 known difference of potential of a portion of A B. This is done by joining Q, 

 the middle point of A B to D, and P, a sliding contact on A B, to C, through the 

 instantaneous contact-maker and a sensitive, dead-beat D'Arsonval galvanometer. 

 When P is so adjusted that there is no deflection of the galvanometer, the poten- 

 tial difference between C and D is the same as that between P and Q, and the 

 latter is proportional to the distance P Q. If the current at the instants of con- 

 tact is from C to D, then the potential of C is higher than D, and P will be on the 

 left of Q ; if the current is from D to C, P is on the right of Q. In either case 

 the strength of the current is proportional to the difference of potential between 

 C and D, and therefore to the distance P Q. At each step of the brush contact is 

 made at a later instant in the period of the wave ; the current has a different 

 value, and hence P must be moved to keep the galvanometer deflection zero. The 

 spiral being of constant diameter, and uniformly wound, these distances P Q give 



