To study a fluctuating copper arc in air with gap length 5 mm and direct current 5 A, spectral-image photographs of which are instantaneously taken by camera shutter in the wavelength range of 4000-6000Å, using slitless stigmatic spectrograph. The space-resolved blackness of each spectral-image photograph is converted to spectral-line intensity by using the N. B. S. standard lamp.The electric field strength at the center of arc column is measured from the relation of arc voltage vs. gap length. From these data, the radial and axial distributions of arc temperature, copper vaper density, electron density, electron current density and electric field strength are calculated. The axial electric field strength is integrated to be a potential of arc column of about 30 V, which is good agreement with that measured independently by the potentiometer.