Two Lecture Demonstrations in Physics

Abstract

(1) Two conducting wires or rods about five feet in length are stretched in a wedge shape, the gap being one-half inch at the bottom and three-quarters of an inch at the top. The upper three feet of the wires are covered with a glass tube. It is well known that if an induction coil is connected to the two wires, the resultant sparks will be carried up the wires by the rising ions; hence the spark will seem to climb the wires. When the spark reaches the top of the glass tube, the ions go out of the end, so that the spark begins again at the bottom. If now the top of the glass tube is tightly corked, the spark when it enters the tube will travel more slowly than along the lower part of the wires, and when it reaches the cork, a layer of ions will stay there, so that the spark, instead of returning to the bottom, will remain just under the cork. With the cork removed, an air blast down the tube will either prevent the spark from climbing or will blow it down the wires.