Four Probe Measurement

1. Probes Arrangement It should have four individually spring-loaded probes. The probes should be collinear and equally spaced. The probes mounted in a Teflon bush, which ensure a good electrical insulation between the probes. A Teflon spacer near the tips should be provided to keep the probes at equal distance. The whole – arrangement should be mounted on a suitable stand and leads are provided for the voltage measurement.

2. Sample: Germanium crystal in the form of a chip (10X9X0.5mm)
3. Oven: Small oven for the variation of temperature of the crystal from the room temperature to about 200°C (max.)

4. Four Probe Set-up: The set-up should consist of three units in the same cabinet.
   
   

(i) Multirange Digital Voltmeter: In this unit, intersil 3½ digit single chip A/D Converter ICL 7107 to

be used. Specifications:

Range: X1 (0-200mV) & X10 (0-2V)

Impedance: MW

Resolution: 100μV at X1 range

Display: 3½ digit, 7 segment LED (12.5mm height) with polarity and decimal indication

Accuracy: ±0.1% of reading ±1digit

Overload Indicator: Sign of 1 on the left & blanking of other digits

(ii) Constant Current Generator. Specifications:

Open Circuit Voltage: 18V

Current range: 0-20mA

Resolution: 10μA

Accuracy: ±0.25% of the reading ±1 digit

Load regulation: 0.03% for 0 to full load

Line Regulation: 0.05% for 10% changes

(iii) Oven Power Supply Suitable voltage for the oven is obtained through a step-down transformer

with a provision for low and high rates of heating. A glowing LED indicates, when the oven power

supply is ‘ON’.