1878.] Mr. W. Spottiswoode on Stratified Discharges. 61 



battery for the purpose of testing the instrument, the same firm con- 

 structed for me condensers of which the following are the particulars. 

 Each condenser is contained in a box, and has a capacity of 13"8 micro- 

 farads, subdivided into ten sections, each section containing forty sheets 

 of tinfoil, 18 in. X 13 in., insulated from each other by eight sheets of 

 parafined tissue paper. The superficial area of foil in each box is 1,300 

 square feet, and that of parafined paper 14,166 square feet. It was 

 found that these condensers could easily be charged with a 4-inch 

 induction coil, worked by two Grove's, or even bi-chromate, cells. A 

 much smaller coil would certainly suffice if the coil were made with a 

 thick secondary, since \- to f-inch sparks are all that are required. In 

 order to charge the condenser, one terminal of the coil was carried to 

 outside of the condenser, and the other to the other with an inter- 

 vening air spark. The object of the air spark was twofold : first to 

 ensure that the tension of the electricity was sufficient to give the re- 

 quired charge to the condenser, and, secondly, to prevent the latter 

 from discharging itself back through the secondary of the coil. After 

 some trials, it was found that the air spark might, with great advan- 

 tage in steadiness of action, be replaced by a vacuum-tube which offered 

 sufficient resistance : such, for instance, may generally be found among 

 those prepared for spectrum analysis, although these differ very widely 

 in resistance. Lastly, the condenser was furnished with a safety dis- 

 charger, consisting of a brass sphere and a point adjustable in distance 

 from one another, so that the condenser might discharge itself at a 

 suitable tension ; i.e., before the tension rose high enough to break 

 down the insulation of the plates. 



The discharge through the vacuum tubes on which experiments were 

 being made was effected either by leading the two sides of the con- 

 denser directly to the terminals of the tube ; or more often by leading 

 one direct, and the other through the intervention of a resistance coil, 

 such as was described in the Proceedings of the Boyal Society for 1875, 

 pp. 461 — 2. By altering the length of the resisting column as the 

 tension in the condenser declined, the charge could be delivered through 

 the tube at any required rate. 



By this arrangement a steady stratified discharge can be maintained 

 for one, two, or more minutes, according to the nature and pressure of 

 the gas contained in the tube. In one case, with a nitrogen tube of 30 

 inches in length and 2 in diameter, a special fixed phase was maintained 

 for upwards of five minutes with one of the boxes above mentioned. 



Speaking in general terms, the same connexion between resistance 

 in the circuit and the flow of the striae as had previously been noticed 

 with the induction coil and rapid contact-breaker (Proceedings of the 

 Boyal Society for 1875, pp. 458 — 9) was observed with this method; 

 but the phenomena were exhibited with greater distinctness, and 

 could be examined more at leisure. 



