1892.] to produce a Spark in Air at different Pressures. 109 



The apparatus was set up, and the observations tabulated below 

 were made in April, 1892. These observations were confined to 

 pressures lower than 50 mm. In each case the two pairs of plates 

 had air-gaps of very different length, so that each series of 

 observations amounted to a determination of two separate potential 

 difference and spark-length curves. The sparks were separately 

 examined, until pressures were reached at which the potential differ- 

 ences necessary for discharge became nearly equal, and then the two 

 pairs of plates were simultaneously connected to the cells. The 

 crossing of the potential difference curves was thus directly verified, 

 a slight lowering of the pressure transferring the discharge from the 

 shorter to the longer air-gap. 



The plates used in this experiment were, all four, plane and 

 polished. Two were of brass, one of copper, and one of zinc. When 

 the different plates were interchanged between the long spark-gap 

 and the short, no difference was observed in the potential difference 

 required for discharge at given difference and air-pressure. That 

 there might be no doubt as to whether the discharge was passing 

 actually between the plates, or was taking a longer path from edge to 

 edge, the edges of the plates were only very slightly rounded off. At 

 low pressures there was considerable tendency to a brush discharge 

 from the edges, but this was considerably reduced by an extremely 

 thin coating of shellac varnish. 



The results given below are those of three single series of observa- 

 tions made on April 21, 22, and 23, respectively. All of these were 

 repeated on the same or other days, and in all cases there was almost 

 exact agreement. For convenience of reference these series are 

 called A, B, and C, the suffix 1 referring to the left-hand, and the suffix 

 2 to the right-hand, spark-gap. As in the former case, the tables 

 give corresponding pressure, potential difference in volts (Y), and 

 electrostatic force in C.G.S. units (F). Curves, similar to those given 

 in fig. 2 and 3, representing potential difference and pressure, and 

 electrostatic force and pressure, are also given in figs. 6 and 7. 



