TRANSACTIONS OF SECTION A. 573 



accurate values of the current. Using these distances as ordinates, and the 

 corresponding angular positions of the brush on the revolving contact-maker as 

 abscissas, a curve of current can be plotted, its scale being, of course, determined 

 by the value of the constant resistance C D through which the current passes, 

 and the value of the constant potential difference betw^een the ends of the 

 solenoid. 



But to eliminate the labour of taking scale readings and plotting the curves it 



Fig. 2. 



is arranged to plot them out automatically as the successive settings of P along 

 the solenoid are made. A pautagraph has one end fixed at P', and the other 

 attached to the sliding contact P. The bar E G of the pautagraph carries a 

 printing point F. As P is moved to and fro upon the solenoid, F travels to and 

 fro along a b, parallel to A B. And since P'/ is one-fifth of P'P, the excursions 

 of F are one-fifth of those of P. When each setting of P has been made, the 

 printing point F is depressed by an electromagnet, and a dot is made upon a sheet 

 of cross-section paper underneath. Between the successive settings the paper is 

 advanced a step perpendicularly to a b. In this way a current curve is plotted to 

 a known scale, and by repeating the process with other currents through C D (as, 

 for example, the secondary current, due to the induction of the first one, or au 

 electromotive force current) several waves may be delineated on the same sheet, 

 each to a known scale, and their relative phases shown by their relative positions. 



In order to advance the paper conveniently by equal steps it is wrapped upon 

 a cylinder, which is advanced step by step by a ratchet wheel and gearing, actuated 

 by an electromagnet, precisely as the contact-maker is operated. These two 

 electromagnets, and that on the pautagraph, are controlled by the same key, and 

 all work together. Closing the key causes the steel point F to be thrown down 

 upon a type-writer ribbon, which prints a point on the paper beneath ; it also 

 causes the armatures of the two driving magnets to be attracted, and the pawls to 

 slip back over one or more teeth of the ratchet, until the armatures strike their 

 respective stops. Breaking the circuit allows the printing point to be lifted by a 

 spring, and the armature of the driving magnets to be drawn back to their initial 

 positions by their springs. The two pawls at this time advance their respective 

 ratchets ; one advancing the brush of the contact-maker, and the other the 

 cylinder carrying the record sheet, ready for a second point. The sliding contact 

 P is moved by means of a cord passing over pulleys at the ends of A B, and wound 

 over a small drum underneath the solenoid, the drum being turned by means of a 

 large milled head. The operator turns this drum with one hand, and closes the 

 key with the other, keeping his eye constantly on the galvanometer scale. With 

 a quick, dead-beat galvanometer the settings are made very rapidly, and it is no 

 unusual performance to print off a curve at the rate of twenty or more points per 

 minute. 



If the curve closely approximates to a sine curve, and there are no high 

 harmonics present, the points may be further apart, and the ratchet of the record- 

 ing cylinder advanced four or six teeth at a time. If there are upper harmonics. 



