148 Mr. R. F. Earhart on the Sparking Distances 



theories have been offered to explain this phenomenon t 

 among them the hypothesis that a certain amount of energy 

 is necessary to tear asunder an air-film which may surround 

 the surfaces in question, the assumption being that, in the 

 neighbourhood of bodies, the air possesses a more or less 

 definite structure, and that in this condition the dielectric 

 properties of air differ from those which it possesses in the 

 ordinary gaseous form. A much more complete series of obser- 

 vations on the spark potential were undertaken by Dr. Liebig 

 in 1887 (Phil. Mag. vol. xxiv. p. 106). In these experiments 

 the distances were varied from *0066 cm. to 1*144 cm. The 

 methods of measuring both distances and difference of potential 

 were similar to those employed by Lord Kelvin. 



Object. — It seems desirable to obtain measurements for very 

 thin strata of the dielectric, and if possible to ascertain the 

 potential necessary to break down the air-film itself. To 

 accomplish this, a much more delicate method of measuring 

 the thickness of the separating medium is imperative if the 

 limiting value is to be approached. The modern form of 

 the interferometer is particularly suitable for measuring small 

 distances, and may be readily adapted to this experiment. 

 The conditions to be fulfilled are extremely simple : we are 

 required, 



1st : To measure a small distance accurately ; 

 2nd : To measure differences in potential accurately. 



To fulfil the first condition. Consider two plane parallel 

 surfaces in contact. Let one of them be rigidly attached to' 

 a fixed support. Let the other be mounted on a movable 

 carriage which bears one of the mirrors of an interferometer.. 

 Upon separating the surfaces, a succession of fringes (pre- 

 ferably circular) will move across the field of view. By 

 counting the procession of fringes, the distance through 

 which the carriage is moved, and therefore the distance of 

 separation of the plates, will be given directly in terms of 

 wave-lengths of light. 



It may readily be shown that a passage of one fringe 

 across the field corresponds to the movement of the carriage 

 through a distance of half a wave-length. Again, since 

 fractions of fringes can be measured readily to one-tenth of a 

 fringe, and since the wave-length of sodium light is approxi- 

 mately equal to *59 yLi, we shall be enabled to measure distances 

 of "03//. if desired. Any departure in the motion of the 

 movable plate from a plane not parallel to itself will be 

 indicated by the appearance of the circular fringes. 



Experience has shown that it is practically impossible for 



