Bridge for Measuring Small Time Intervals 



By J. HERMAN 



Synopsis: A bridge circuit for measuring time intervals from about one 

 ten-thousandths of a second up to several seconds is described and its oper- 

 ation explained. The device is fairly accurate and easy to operate and gives 

 the results of measurements in fractions of a second directly. Its cali- 

 bration can readily be determined mathematically since it is dependent only 

 upon the values of certain capacities and resistances used in the measuring 

 circuit. 



TO the large family of measuring devices making use of certain 

 principles of electrical balance there has recently been added a 

 new member. This measures the elapsed time between the opening or 

 closing of one set of contacts, and the subsequent opening or closing of 

 another set of contacts, the agency employed for operating the contacts 

 being immaterial. The particular form of the device described below, 

 was designed primarily for use in adjusting the operating and releasing 

 times of the voice-operated switching relays at the terminals of the 

 transatlantic radio telephone circuit. In this form or with minor 

 changes, it is applicable to the measurement of intervals of time in the 

 operation of a large variety of other types of apparatus. 



The new time measuring device is simple and easy to operate, its 

 operation consisting merely of opening and closing a key repeatedly 

 and securing a balance by observing a meter. The balance is secured 

 by turning one or more dials which are calibrated in fractions of a 

 second. 



A range of measurements extending from about one ten-thousandths 

 of a second up to several seconds is readily obtainable and an accuracy 

 of measurement to within ± 1 per cent can probably be realized over 

 the greater part of this range if sufficient care is taken in the design of 

 the circuit and the selection of the apparatus. In a fairly rugged type 

 of bridge, now in commercial service (See Figs. 3 and 4), which covers the 

 range from one ten-thousandth of a second to one second, and in which 

 little attempt was made to secure a high degree of sensitivity, the 

 results of measurements are accurate to within dz 5 per cent for time 

 intervals down to about five thousandths of a second. For time inter- 

 vals below this value the accuracy decreases rapidly, due partly to the 

 fact that the smallest step provided for on the dials is one ten-thou- 

 sandth of a second and partly due to the effect of variations in the 

 operating time of the relays used in the bridge. 



Because of its simplicity and accuracy, the bridge is especially 

 valuable for making a series of time measurements to determine the 



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