Analysis Used in Submerged Body Research 411 
determining easily whether any changes other than hydrodynamic have occurred in the total 
system and it eliminates the need for subtracting arbitrary readings on each channel to ob- 
tain the net readings. The pen position adjustment is accomplished by a potentiometer 
which is connected in parallel with the feedback potentiometer. 
A span or sensitivity adjustment is provided to establish the calibration of the digital 
indicator in terms of the load on the gage. The span control is a potentiometer which is 
placed in series with the part of the circuit that goes with the feedback potentiometer slider. 
Thus the unbalanced voltage resulting from displacement of the gage appears across the 
span potentiometer as well as any other resistors placed in series with it. The range of 
sensitivity varies from nearly zero to an amount somewhat in excess of that required to ac- 
commodate the maximum sensitivity of all the types of transducers used in the tests. The 
span potentiometer has a calibrated index and can be locked into place. However, since the 
control units and gages can be interchanged, the span potentiometer setting is not suffi- 
ciently accurate. Therefore a system for setting sensitivity which is independent of the 
transducer movement is provided. This “span check” is made by applying a step signal to 
simulate an actual transducer change. To do this, a precision resistor in the control box is 
shunted across one gage coil. The resulting span check reading on the digital corresponds 
to the given sensitivity and to the nominal setting on the span potentiometer. 
Span control settings and span checks are usually established with the modular force 
gage or roll balance mounted on a calibration stand. Since all components of the measure- 
ment system are linear, the settings are determined on the basis of that required to give a 
reading of exactly 1000 counts on each channel for some predetermined load. As mentioned 
earlier, the usual sensitivity is 200 pounds for 1000 counts; however, sensitivities of twice 
or one-half of this amount are used from time to time, depending on the range of loads en- 
countered in the test. The span checks for each calibration are recorded in a log book. 
These values have been found to hold true for any particular gage and control box combina- 
tion over a period of years. 
The signal coming from the gages is a fluctuating one even in steady-state tests. This 
is due largely to carriage vibrations which are transmitted to the model through the rigid at- 
tachment. A filter is provided in the control box to smooth this signal to obtain one steady 
value at the digital indicators. The filters are made up in steps so that only the amount 
needed for smoothing is used without needlessly sacrificing speed of response. The filter 
switch connects successive values of capacitance between the span potentiometer slider 
and one side of the feedback potentiometer. The polarity between these two points is al- 
ways the same, so that electrolytic capacitors of reasonable size can be used. Since this 
capacitance is outside the servo feedback loop, it introduces no instability. 
The recording and measuring equipment for the dynamic tests uses the same control 
unit, 400-cycle power supply, and digital readout or Brown recorder system. The distin- 
guishing features are: the introduction of the signal amplifier, the force component separa- 
tor, the integrating amplifier, the multiganged sine-cosine potentiometer, and the integrate 
timer and function relays. These components are shown by the lower leg of the block dia- 
gram of Fig. 22. They become part of the measuring and readout system when the selector 
switch is thrown from static to dynamic. The selector switch also selects the type of read- 
out; that is, digital or Brown recorder. The digital system, however, is represented in 
Fig. 22. 
The signal amplifier shown by the block diagram is a high-gain transistorized amplifier 
that provides accurate amplification of signals in millivoltages over the frequency range from 
direct current to 20 kilocycles. Some of the principle features of this amplifier are: high 
