16 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



1-14. Thickness Determination from Measurements of 

 Electric Breakdown Voltage 



The thickness of thin homogeneous insulating layers, such as oxide 

 layers electrochemically produced on aluminum or its alloys, can be 



AVWV 



B 



zz, 



T)/d 



~ 



determined from measurements of 

 the electric breakdown voltage. The 

 system employed for this purpose is 

 illustrated schematically in Fig. ( 1 - 1 ) 

 1 3 and consists of a polished ball B 

 of copper, brass, bronze, or chro- 

 mium-plated steel, about ^in. diam- 

 eter, pressed against the test object 

 T with a force of 1 to 2 kg. An a-c 

 voltage is applied through a safety resistor and is increased until 

 breakdown occurs; the rms value of the voltage is measured. 



Fig. (1-1)13. Thickness gauge based 

 on breakdown-voltage measurement. 



2 4 6 



Breakdown voltage, x 100 volts 



Fig. (1-1)14. Thickness of aluminum oxide coatings as a function of 

 breakdown voltage for three different alloys (from G. Keinath, The 

 Measurement of Thickness, Natl. Bur. Standards Circ. 585, p. 31, 

 1958; by permission) . 



The system has been used for the measurement of layers 1 to 25 /j, 

 thick; the breakdown voltage increases with the thickness and is 

 about 1,000 volts for a 25- [x layer of aluminum oxide. The error is 



