685 
he 
corresponding to charge developed by a piezoelectric gauge, is impressed on 
the coupling circuit and amplifier. In this use, the Q-step eliminates 
the need for an impedance bridge in determining cable response characteristics, 
as well as providing the necessary information in a more useful forme 
4. Transient voltage standards 
The requirements of a voltage standard are that it develop accurately 
known, dependable range of voltage values. It is very desirable also to have 
an internal means of checking these voltages at least approximately. If the 
same circuit is to be used as a generator of transients, it is necessary 
that the transient be of known and reproducible form adapted to calculation 
of response errors, The best single function for such a purpose is a voltage 
step, or Heaviside Unit Function, which jumps from one voltage level to an- 
other in a time ideally zero, practically small compared with any intervals 
involved in the measurement. Although a square wave is an approximation to 
such a step function much used in commercial practice, it has not been em- 
ployed for testing of transient response at this lahoratory because it 
gives no information at frequencies below the repetition rate, and because 
no good way of insuring the accuracy of its amplitude was seen. Instead, 
attention has been concentrated on development of step=-function generators 
that would provide step functions of known displacement either by manual 
control or automatic triggering at regular intervals, and would provide a 
synchronizing pulse preceding the step by an adjustable time interval. 
These generators may be resolved into two parts: a standard voltage source, 
and a switching method for producing the step function. 
(a) Standard voltage sources. -=- The requirements placed on a cali- 
bration voltage standard are that it remain reasonably constant over time 
intervals between checking and that it be capable of supplying adequate 
power to the external circuit without disturbing the calibration -- in 
other words, that it permit low-impedance output from the source, It is 
usually difficult to satisfy both requirements simultaneously; most sources 
compromise between stability and power capability. 
(i) Dry cells. The simplest standard voltage source is a dry cell 
operating under constant but low current drain. A circuit employing a 
Burgess type FA 13-volt cell, operating with a drain of 1 ma requires 
recalibration once every two week&. This is quite satisfactory if the 
moderately high output impedance can be tolerated, 
