Variables measured directly in the string apparatus include
the forcing phase, 
, and the displacement amplitudes
of the string, x(t) and y(t). The phase is measured directly
from the function generator that provides the sinusoidal current
to the wire. The string displacement is measured from the optical
detectors that record the horizontal and vertical displacement at
a fixed point along the wire. In addition, the
wire's velocity can be measured by sending the amplitude displacement
signal through a differentiator , a circuit that takes
the analog derivative of an input signal.
Many approaches are possible for
constructing an experimental Poincaré
section .
The particular approach taken depends on both the type of system and
the equipment at hand. Here, we assume that the lab is stocked with a
dual-trace storage oscilloscope and some basic electronic components.
A different approach might be taken, for instance, if we have access to
a digital
oscilloscope either in the form of a commercial instrument or a
plug-in board
to a microcomputer.