8 Trans. Acad. Sen. of St. Louis. 



cliinery is different in the two cases. Cakes of ice float- 

 ing on water have as little to do with the athletics of a 

 foot race, as air molecules have to do with the flow of 

 electricity in a power circuit. 



The figures of Plate III show clearly the difference 

 between the two terminals of an influence machine. They 

 are camera photographs of a discharge between termi- 

 nals, having a coj^per plate midway between them. The 

 Leyden jars have been removed from the machine. The 

 discharge is a brush discharge. Only faint disruptive 

 discharges now and then occur. The copper plate has a 

 hole in it, which in Fig. A is opposite the positive term- 

 inal. The negative terminal emits a discharge 

 into the air in front of it. This air is then 

 urged across the dark space to the copper plate. 

 The drainage column from the positive terminal 

 also reaches the copper plate. Air molecules in the hole, 

 in the presence of the negative terminal, also lose their 

 negative charge to the drainage column. The drainage 

 cohimn thus elongates through the hole until it reaches 

 the negative glow. In Fig B, we have precisely the 

 same arrangement, excepting that the hole is in front 

 of the negative terminal. The drainage column does 

 not then reach the hole. Molecules overloaded with neg- 

 ative fluid do reach and pass through the hole, but the 

 copper plate shields the positive knob from the influence 

 of the negative terminal. The negatively electrified par- 

 ticles which pass through the hole induce a small drain- 

 age into the positive knob as seen at its lowermost point. 

 This inflow to the knob ceases if the hole is covered with 

 a thin sheet of paper. The other portion of the dis- 

 charge is unchanged. In Fig. C of Plate III the term- 

 inals are in line but the copper plate is displaced. The 

 drainage column which penetrates the hole is partially 

 intercepted by a small disk of copper mounted on a glass, 

 support. This copper disk obstructs the drainage col- 

 umn which reappears at its edge. 



