208 TROWBRIDGE. — IIIGII ELECTROMOTIVE FORCE. 
lensity of vapor, together with a Doppler effect, might be important elements in 
pro<h icing small displacements. 
It is only when the metallic terminals are close together — three to four centi- 
meters or less — that those pi 
Indeed, even with sparks of 
feet in length in air, thus indicating a very high electromotive force, no spectroscopic 
of the metals of the sp 
s 
be obtained at a distance of three 
the initial pressure along the path of the spark would be 660 atmospheres. 
When terminals of different metals are employed in capillary tubes of glass or 
quart/,, and are separated four or five millimeters, complicated phenomena result from 
powerful condenser discharges through the rarefied gases contained in these tubes. 
All specimens of glass which I have tried — soft German glass, lead glass, Borsilicon 
glass, or Jena glass - give broad bands due to silica; lead *}*M gives, in add 
d lines. Jena glass gives a very strong line of boron at wave-length, 3451.49. 
These hnes and bands are obscured by a continuous spectrum. 
rminals which I have used may be called 
The narrow cap 
furnaces, in which there is no 
permanent product or permanent d 
ipectra which we observe do not reveal all 
cap 
«-f 
» 
inch' from such terminals. The particles of the metals are apparently too heavy 
to bo projected to any considerable distance, and the spectrum obtained is that of 
heated air. 
In the study of displacements seen in comparison of spark and arc spectra, the 
phenomenon I have observed may have importance ; for, by changes in position of 
the metallic electrodes and change in pressure of the rarefied gas, one'can vary the 
conditions over a wider range than when the study is made in atmospheric air at 
atmospheric pressure. It is evident that when the terminals are within 1 cm. apart, 
powerful condenser discharges approach the condition of the voltaic arc. The 
pressure produced by powerful condenser discharges in capillary tubes is perhaps an 
element in producing the broadening of the lines toward the red. 
In Professor J. J. Thomson's "Conduction of Electricity through Gases," 1906, 
p. 516j the subject of pressure in the spark is considered from the point of view of the 
kinetic energy given to the ions, and the author calculates that this energy of a spark 
1 cm. long in air at atmospheric pressure, from a condenser of 1000 cm. capacity 
charged to 30,000 volts, if distributed throughout 1 c.c. of gas, would increase the 
pressure by 6.6 atmospheres. When confined to the very much smaller volume 
traversed by the spark, the pressure would rise to enormous values. To take T J T of 
1 c.c. as the volume of gas traversed by the gas, which would be a large overestimate, 
\ 
