HERTZIAN ^YAVE WIRELESS TELEGRAPHY. 363 



found that if a certain voltage between metal surfaces would yield a 

 spark four inches in length, at the ordinary pressure of the air, if 

 the spark balls were enclosed in a cylinder, the air round them com- 

 pressed at 50 lbs. per square inch, the spark length for the same poten- 

 tial difference of the balls was only one quarter of an inch, or one 

 sixteenth of its former value. 



The writer has also made experiments with an apparatus designed 

 to study the effect of compressed air round the spark gap. The 

 experimental arrangements are as follows: A ten-inch induction coil 

 has one of its terminals connected to the internal coating of a battery 

 of Leyden jars. The external coating is connected through the pri- 

 mary coil of an oscillation transformer with the other secondary ter- 

 minal of the coil, and these secondary terminals are also connected 

 to a spark gap consisting of two brass balls enclosed in a glass vessel 

 into which air can be forced by a pump, the air pressure being meas- 

 ured by a gauge. The balls in the glass vessel are set at a distance 

 of about three millimeters apart. The secondary circuit of the oscil- 

 lation transformer is connected to another pair of spark balls, the 

 distance of which can be varied. 



Suppose we begin with the air in the glass vessel containing the 

 balls connected to the secondary terminals of the induction coil, which 

 may be called the secondary balls, at atmospheric pressure, and create 

 oscillatory discharges in the primary coil of the oscillation trans- 

 former, we have a spark between the balls, which may be called the 

 tertiary balls, connected to the secondary terminals of the oscillation 

 transformer. If the secondary balls are placed, say three millimeters 

 apart, the air in the glass vessel enclosing them being at the ordinary 

 atmospheric pressure, then with one particular arrangement of jars 

 used, a spark twenty-five or twenty-six millimeters long between the 

 tertiary balls will take place. Suppose then we increase the pressure 

 of the air round the secondary balls, pumping it up by degrees to 10, 

 20, 30, 40 and 50 lbs., per square inch, above the atmospheric pressure. 

 We find that the spark between the tertiary balls will gradually leap 

 a greater and greater distance, and when the pressure of the air is 

 50 lbs. per square inch, we can obtain a fifty-millimeter spark between 

 the tertiary balls, whereas when the air in the glass vessel is at atmos- 

 pheric pressure, we can only obtain a spark between the tertiary balls 

 of half that length. 



This experiment demonstrates that the effect of compressing the 

 air round the secondary terminals of the induction coil is to greatly 

 increase the difference of potential between these balls before the spark 

 passes. In fact, it requires about double the voltage to force a spark 

 of the same length through air compressed at 50 lbs. on the square 

 inch that it does to make a spark of identical length between the 



