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BELL SYSTEM lECIISICAL JOURNAL 



less thiin the accidental irregularities in a contact obtained from the 

 escapement wheel even excluding errors due to eccentricit\-. 



This method of obtaining an electrical impulse from a clock is of 

 great \alue as it may be ap]>lied to practicalK' an\- clock which may 

 not have any other method of producing impulses. 



The second type of error is due to \ariations in the rate of the clock. 

 Two fundamental requirements in the design of an accurate clock 

 are that the impulse delivered to the pendulum be symmetrical about 

 the mid-point of its swing and be not subject to irregularities in 

 magnitude or duration, and that the pendulum be free at all other 

 parts of its swing. These requirements arc fairly well met in the 



l"ig. S — Circuit of Pli<)to-t'!c<tric ("ell Urixc of St.iiulard Clork 



present clock. However, the magnitude of the impulse depends on 

 the constancy of the voltage of the driving ceil which is a single 

 primary cell of rather small size, and the duration of the impulse may 

 be somewhat variable due to the possible eccentricity of the escape- 

 ment wheel and due to the method of operation. The pendulum 

 too, is not entirely free from constraint at any part of its swing. 

 These errors may all be avoided or at least considerably reduced by 

 the use of the impulse obtained from a photo-electric cell to drive the 

 clock and by the use of a more constant source of primary voltage. 



The use of this t\'pe of dri\'c has accordingly lieen investigated in 

 connection with this clock. It is obvious, since the driving impulse 

 is one of attraction between the coil and the bar carried by the pen- 

 dulinn, that it must be exerted only once per second, that is, when the 

 |)en(hilimi is entering the driving coil and not when it is returning. 

 The circuit used is shown on Fig. 5 and operates as follows: 



When the relay R2 operates the first time in the second, it closes 

 the circuit through the winding Wx of the relay Ra and through relay 

 Rj. This operates the relay Ri and closes the circuit through the 

 driving coil of the clock. The current through the one winding of 



