Observation of Higher Members of the Balmer Series and the Hydrogen Spectrum of a Long Discharge Tube

R. W. Wood, “Hydrogen Spectra from long vacuum tubes.” Philosophical Magazine, s. 6, vol. 42, p. 729, 1921.

Until now, by laboratory methods it had been possible to photograph no more than the first 12 members of the hydrogen Balmer series. The higher members were found only in the hydrogen spectrum of the light of nebulae. Wood constructed a long discharge tube in which it is possible, spectrographically, to detect the presence of the first 20 members

hydrogen Balmer series. This tube has the form shown in the figure. The total length of the tube is about 2 meters. Observation of the serial spectrum is carried out mainly in the central bend of the tube. The electrodes are cylindrical and are made of thin aluminum. Hydrogen, obtained by the electrolytic method, is introduced into the tube through a capillary (as shown in the figure) without a dryer, since the presence of water vapor is a necessary condition for the appearance of the many-line serial spectrum. The tube is supplied by a small transformer at 25000 volts; the current in the primary winding, at a voltage of 110 volts, can reach 30 amperes. With weak currents in the tube, in the central part several lines of the Balmer spectrum are observed, and the intense secondary spectrum of hydrogen (caused, according to current views, by diatomic hydrogen molecules). As the current is increased, the intensity of the secondary spectrum in the central part of the tube decreases, reaches a minimum, and with a further increase in the current strength again increases. The intensity of the serial spectrum increases linearly with increasing current strength in the primary winding of the transformer. In the bent ends of the tube the secondary spectrum remains very intense all the time.

Figure labels: to the pump; 30 cm; 80 cm; 80 cm; capillary.

As already indicated, Wood succeeded in photographing 20 lines of the Balmer series. The 20th line is apparently the limit, since the complex secondary spectrum, increasing again after passing through the minimum (about 5 amperes), makes it difficult to distinguish the weak lines of the series. The relative intensity of the lines of the series is as follows: 1st line 1, 6th line about \(\frac{1}{60}\), 14th line \(\frac{1}{15000}\), 20th line \(\frac{1}{300000}\). In the case of nebulosities the 14th line has an intensity \(\frac{1}{50}\) of the \(H\alpha\) line. Wood determined the wavelengths of the new members of the Balmer series observed by him, making use of the iron spectrum for comparison. The results are given in the table.

For comparison, the table gives figures for the same lines, determined by Dyson and Evershed for the hydrogen solar lines.

In dry hydrogen the Balmer lines almost completely disappear in the central part of the tube even at large currents; instead, a group of new, very bright lines of unknown origin appears, approximately in the interval \(410\text{—}416\,\mu\mu\). Wood suggests that these lines may be due to a compound of silicon with hydrogen. If a condenser is introduced into the circuit of a tube filled with dry hydrogen, then the intense Balmer lines appear again. The explanation is that including a condenser in the circuit is equivalent to feeding the tube with currents of thousands of amperes of short duration.

Wood describes a number of other interesting features of the operation of the long discharge tube, on which we shall not dwell here, referring the reader to the original. The paper also contains detailed technical instructions.

The form of the tube and the conditions of its operation were found purely empirically. The essence of the observed changes in the spectrum remains as yet unexplained.

S. Vavilov.