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



vations of Dr. Liebig, previously mentioned in this article, 

 are perhaps more complete for relatively large distances than 

 those given in any published table. Dr. Liebig has two values 

 which fall within the limit of this experiment. Reducing his 

 observations to the scale here used, they show a very close 

 agreement. They are indicated in fig. 3 by a dagger, together 

 with the letter L. 



Effect of Pressure. — Tables II. to V. indicate the values 

 obtained for the pressures indicated. These results are 

 represented graphically in fig. 4. The values obtained 

 indicate that for varying pressures the general form of 

 the curve is the same as that under atmospheric pres- 

 sure, but that after passing a certain limiting distance, 

 the second straight portion of the curve makes a different 

 angle with the X-axis, the inclination depending on the 

 pressure. For pressures greater than one atmosphere, values 

 of the potential up to a distance of three wave-lengths seem 

 to coincide very closely with the values given for one 

 atmosphere. After passing this " turning-point/'' the curve 

 again becomes a straight line. For pressures less than one 

 atmosphere the values indicate that, in the first straight 

 portion of the curve, a smaller potential is required to cause 

 the spark to pass a given distance. The character of the 

 curve would lead us to believe that for very low exhaustions 

 the resistance of the air-film, if such does exist, becomes 

 much weakened, and that for minute pressures we should 

 have a curve lying close along the Y-axis. The discharge 

 through a Geissler tube is probably of this character. 



In obtaining the readings under pressure other than one 

 atmosphere, the entire instrument was enclosed in an iron 

 box provided with suitable windows. The movement of the 



Table II. — Air, 



40 cm. Pressure. 



Distance in 



Potential in 



Distance in 



Potential in 



Wave-lengths. 



Volts. 



Wave-lengths. 



Volts. 



10 



52 



150 



328 



1-5 



75 



19-0 



35(5 



1-5 



82 



21-0 



360 



2-0 



104 



33'5 



412 



2-5 



146 



40-5 



464 



2-5 



150 



540 



496 



30 



152 



67-0 



552 



4-0 



170 



750 



592 



4-5 



204 



1250 



660 



5 



304 



178-5 



720 



5-5 



320 







