  We explore the possibility of using free (as opposed to driven) oscillations
to verify wire tension in large, open-frame multiwire planes. Using
finite-element simulation we predict the signal when two wires are set in
motion by applying and holding a DC voltage between them (electrostatic
excitation). The resulting current signal is marginal. However, the program
relates voltage, oscillation amplitude and signal, and it is easy to get enough
amplitude (of order microns) by gently tapping the wire frame. We perform an
end-to-end experimental test. Four 1.5m wires, at various tensions around 5N,
are mounted in an aluminum channel. They are connected to a low-noise, balanced
current-to-voltage preamp designed for good common-mode rejection. The 150V
bias, from 9V batteries in series, is built into the preamp. Data are taken
with a Tektronix 1202B scope, stored on a USB flash drive, and transferred to a
laptop computer for analysis. Ten measurements, each with 2500 samples spanning
1s, are taken for each of the three wire pairs. The wires are excited by
tapping the frame with a small mallet. Because the wire tensions are different,
the waveforms exhibit beats. The data are analyzed by a program which
decomposes the waveform step by step to extract the base and beat frequencies
and thereby, two resonant frequencies per wire pair. Finally, those frequencies
are disambiguated to obtain the frequency for each wire, which is then
converted to a tension. The four tensions are then found mechanically by
measuring the g/mm required to deflect each wire at its midpoint. The two
values of tension agree well. We conclude that the method verifies tension to
approximately 2% (1 sigma) in a little over 1s for 1.5m wires, and would be
faster for shorter wires.

Instrumentation and Detectors