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By M Naidu

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14). Since the usual gap distance used for glow discharge tubes of smaller sizes is about 3 mm, the gas pressure used in case of hydrogen will be or, § = 25 torr 45 and in the case of air it will be -^ =15 torr. 3 : What will the breakdown strength of air be for small gaps (1 mm) and large gaps (20 cm) under uniform field conditions and standard atmospheric conditions? 22 + 6 08 ^ . 4 cm between the plane electrodes. 5 x 1(T9A. Calculate Townsend's primary ionization coefficient a. Solution: The current at the anode / is given by /= / O exp(arf) where /Q is the initial current and dis the gap distance.

8 TIME LAGS FOR BREAKDOWN In the previous section, the mechanism of spark breakdown is considered as a function of ionization processes under uniform field conditions. But in practical engineering designs, the breakdown due to rapidly changing voltages or impulse voltages is of great importance. Actually, there is a time difference between the application of a voltage sufficient to cause breakdown and the occurrence of breakdown itself. This time difference is called the time lag. The Townsend criterion for breakdown is satisfied, only if at least one election is present in the gap between the electrodes.

Of the above properties, dielectric strength is the most important property of a gaseous dielectric for practical use. The dielectric strength of gases is comparable with those of solid and liquid dielectrics (see Fig. 22). ' These are shown in Fig. 23. This feature of high dielectric strength of gases is attributed to the molecular complexity and the high rates of electron attachment (see Sec. 7). 2. Gas pressure (kg/cm2) Fig. 23 Breakdown strength of insulating gases for 75 cm diameter uniform field electrodes having 12 mm gap From the figures and the table, it can be seen that SFe has high dielectric strength and low liquification temperature, and it can be used over a wide range of operating conditions.

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