THE RESONANT RECORDER 15 



itself is maintained in a state of persistent vibration by the 

 usual electro-magnetic arrangement. I shall for the sake 

 of convenience refer to this reed-interrupter as the Coercer ; 

 and the writing-index simply as the vibrating-recorder or 

 Vibrator. The reed was at first purposely selected of too 

 great a length, so that the natural frequency of the coercer 

 should be slower than that of the vibrator. The free end 

 of the coercing reed carries a platinum wire which, dipping 

 into a cup of mercury, completes the electric circuit. The 

 other end is clamped, and by shifting the clamping-point 

 the vibrating length of the reed is gradually and continuously 

 shortened. This has the effect of gradually raising the 

 vibration-frequency of the interrupting reed. A time soon 

 comes when the frequency of the coercer is exactly the 

 same as that of the vibrator. The latter, which has been 

 hitherto more or less inert, now suddenly breaks out, as 

 foreseen, into very regular and sustained vibrations of large 

 amplitude. For some purposes it is important that the 

 vibration-frequency of the recording-index should have a 

 definite value. The various frequencies most suitable for 

 these researches were 10, 20, 50, 100, and 200 vibrations 

 per second. The exciting reed was previously calibrated 

 by means of frequency-meters, or standard tuning-forks, to 

 give these values. Then, with great expenditure of time 

 and patience, different vibrating-recorders having various 

 standardised frequencies were constructed. For this we 

 have to select fine-steel wires of differing lengths and thick- 

 nesses. The final tuning is accomplished by careful filing 

 or hammering. Fihng the tip of the vibrator raises the 

 frequency ; and hammering of the wire, near the point of 

 suspension, lowers it. A general flattening, along the whole 

 length, tends to maintain the vibration in a definite plane, 

 otherwise the free tip is apt to execute an elliptical vibration. 

 The tuning of the vibrator with the coercer is not very 

 difficult when the vibration-frequency is low, say 10 per 

 second. But when the frequency is high, say 100 or 200 times 

 in a second, an exact tuning is essential. The slightest 



