700 
becpioy 2 
use of a single knob, all electric switching is accomplished for the three 
photographs required for one shot, namely time-scale calibration, Q-step cali- 
bration, and the actual transient. Also included in the panel is a meter which 
measures the gauge cable resistances, a space for the "plug-in" cable com 
pensation networks, and an interlocking firing-switch system which prevents 
the operator from firing the charge when the master switch is in the wrong 
position. 
The circuit for the master control is given in Fig, 28. The design is 
a matter of switching and layout ta give 4 maximum utility and freedom from 
error. 
A master control unit of this type is useful primarily for work in which 
a considerable number of similar operations are to be conducted. In these 
circumstances, the time saving and increase in efficiency due to simplicity 
of operation and freedom from human error are very great, and can perhaps 
be properly appreciated only by those who have to operate a complicated 
array of equipment and by those who have to analyze the records. 
9. General considerations in instrumentation 
A background of four years! experience in design and construction of in- 
struments for recording transients in-the field has shown the importance of 
factors which are of less concern in laboratory work, These considerations 
are doubtless familiar enough to all who have made such measurements, but the 
following discussion is included as a possible help to those who may be 
called upon te plan equipment for field tests. 
Field measurements of underwater explosions must frequently be made under 
adverse conditions, both for the operator and for the equipment. It is there- 
fore important that the equipment function properly under unfavorable com- 
binations of temperature, humidity, and primary power-supply variations, For 
the sake of the operator, it is also important that the necessary controls be 
simple and straightforward, and that proper functioning of the equipment be 
easily determinable. The possible need for repair ines the field with limited 
facilities should also be taken into account. 
A particularly important consideration is the fact that explosions occur 
once and may involve considerable amounts of time, effort, and money. In 
these circumstances, equipment which works 95 percent of the time may be 
worse than useless, 
