For the central portion of
an infinitely long thin rod at steady state,
and Equation 4 becomes,
where T is the uniform central temperature. By measuring V, I,
and T and sample geometry, the electrical resistivity can be calculated
from the relation
In
Eq. (6), A is the sample cross-sectional area, L is the distance
between the voltage probes, V is the voltage drop across the probes,
and I is the current. The total hemispherical emittance is
calculated using the same input plus the sample circumference.
Equation (5) reduces to
In practice the sample diameter (and bulk density) are determined. Then
voltage probes are attached over a central portion of the sample. The
electrical resistivity at room temperature, the effective voltage
probe separation distance, and the appropriate factors (A/L and
PL_) are determined prior to insertion of the sample into the
multiproperty apparatus. This is done with the aid of a knife blade
holder which holds two knife blades a precisely known distance apart.
The sample is connected to a regulated DC power supply in series
with a reversing switch and a precision shunt. The voltage drop across
the shunt, the voltage probes, and the knife blades are measured
when the knife blades are located over the same general region of the
sample as the voltage probes. The current flow is reversed and
the procedure repeated. The measured voltage drops are averaged to
eliminate stray emf effects. The current is determined from the
voltage drop across the standard shunt. The resistivity is
determined from Eq. (6) and the effective distance between the voltage
probe wires is calculated from the relation: 
The computer program RESIST collects the data and averages the measured voltages and outputs A, L, A/L, PLsigma, and rho. The system is evacuated, and the sample heated to a temperature above or equal to the highest test temperature desired. After appropriate aging, the voltage drop, current flow, and temperature are measured, the current flow reversed and the process repeated. The sample temperature is lowered to a new level and measurements repeated. This procedure is followed until the desired temperature range has been covered.
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