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MR. E. G. BILHAM ON THE SPARK SPECTRUM OF 
These conclusions have recently been strongly questioned by Evershed* who has 
deduced a zero pressure and ascribes the solar displacements to Doppler effects. 
The elucidation of the problem depends upon the comparison of the solar displacements 
with pressure displacements of comparable magnitude. It was felt, therefore, that a 
study of the effect of pressures up to about ten atmospheres above the normal was 
likely to yield the most useful results in relation to solar problems. 
2. Apparatus and Method. 
The Pressure-Chamber .—This was made from a T-shaped piece of steam-piping, 
4 inches in external, and 21; inches in internal diameter. The three limbs terminated 
in flanges to which the covers were bolted, leather washers being interposed to make 
the joints airtight. One of the electrodes was fixed in position and well insulated by 
means of a cylindrical block of ebonite. The other electrode was movable so that the 
spark-gap could be adjusted to any desired distance, by turning a nut attached to the 
outside of the cylinder. A thick leather collar acted as a substitute for a stuffing box 
and worked very satisfactorily under the pressures employed. 
The vertical limb of the T was closed by a cover having a glass window 1 inch in 
diameter. The cover could be easily removed and replaced, and was always removed 
while the exposures on the normal spark at atmospheric pressure were being made. 
The apparatus was further provided with a safety valve and pressure-gauge, and the 
pressure was applied by means of a cylinder of compressed nitrogen. The nickel rods 
were cut from sheet nickel, and were held in clips attached to the electrodes. The 
pressure-chamber was supported in a horizontal position on a wooden stand provided 
with levelling-screws. 
Method of Excitation .—In order to obtain a spark of sufficient brilliancy to keep 
the exposures within reasonable limits, a Resonance Transformer, designed by Mr. A. 
Eagle, was used. A motor-alternator supplies an alternating E.M.F. of 50 cycles per 
second to the primary coil, at any desired voltage up to 200, and produces a potential 
difference up to about 100,000 volts at the spark-gap. A large adjustable parallel- 
plate condenser is connected in parallel with the secondary. Since the movable 
electrode was in metallic connection with the case of the pressure-chamber and the 
gas-holder the corresponding secondary terminal was carefully earthed. In air at 
atmospheric pressure a very brilliant spark is produced with a spark-gap of two or 
three millimetres. Under a pressure of eleven atmospheres a gap of about one 
millimetre gave the best results. The behaviour of the nickel spark under pressure 
displayed some points of interest. Although such a small spark-gap was employed, 
the spark was exceedingly brilliant and noticeably whiter in colour than the normal 
spark. The electrodes were rapidly disintegrated, and although the design of the 
* ‘ Kodaikanal Obs. Bulletin,’ No. 36. 
