Thermal expansion is determined using the twin-telemicroscope method to measure the relative displacement of fiducial marks on the sample as a function of temperature. The lower end of the sample is connected to a relatively force and friction-free movable electrode connected through springs to a load cell. The load cell has two strain gauges mounted on it to measure both positive and negative loadings. A load of 5 grams (about 0.01 pounds) is easily detectable. The vertical position of the load cell can be varied through a gear arrangement connected to a rotary gear located on the outside of the vacuum enclosure. Using the rotary gear the loading can be kept near zero so that the sample can expand and contract under essentially no-load conditions.
The twin telemicroscopes are mounted on an Invar bar and can be raised and lowered simultaneously. The telemicroscopes are moved so that the upper telemicroscope remains focused on a particular fiducial mark. This in effect zeroes out the expansion of the sample between the upper electrode and the upper fiducial mark. This displacement of the lower fiducial marks which are located about one inch below the reference mark is measured using the filar eyepiece of the lower telemicroscope. This displacement along with the sample temperature is used to compute delta L/L as a function of temperature.
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