Short-Length Tungsten Arc Characteristics

The method used was a dynamical one. Oscillograms were taken giving the current through the arc and the p.d. across the electrodes (4 mm in diam.) as the latter were uniformly separated. This was accomplished by connecting the cam which separated the electrodes directly to the shaft of the oscillograph drum so that the exposure on the film was made while the arc gap was being lengthened. From the measurements of the oscillograms, curves are plotted giving p.d. as a function of current for constant gap length of from 0.005 mm to 0.4 mm, also curves giving p.d. against gap length for different constant current values from 2 to 12 amp. The intercepts of these latter curves on the voltage axis give the "minimal length" characteristic. It is found that (1) for an arc length 0.4 mm, and for shorter gaps where the current is 4 amp or more, the characteristics are expressed by Nottingham's equation, $E=A+\frac{B}{i^n}$ where $n=1.49\mathrm{}$, (2) the value of $n$ for tungsten fits roughly the linear relation of $n$ to the absolute temperature of the boiling point of the anode material, suggested by Steinmetz; (3) the minimal length characteristic is expressed by $E=13.2+\frac{1.05}{\mathrm{}(i-75\mathrm{})\mathrm{}}$, which makes it possible to compute the constants of an ignition or relay circuit so as to eliminate sparking between tungsten contact points.