“Teena,” Jim said, handing her the earphones which were attached by a long wire to the Geiger counter, “you take these earphones. Now, I’ll pass the probe down close to the base of the cotton bush.” “What does the stick do?” Teena asked. Although Eddie had explained it to her, she seemed to feel that, as a pupil, she should ask some questions to help Jim out. “Stick? Oh, you mean the probe. Actually, it’s called a diode, but probe’s easier to remember. Anyway, the probe is a vacuum tube filled with a special kind of gas. Whenever invisible radioactive particles shoot through the probe and into the gas, the Geiger counter clicks, and the needle on the dial moves forward. The more rays shooting through the probe, the more clicks; the more clicks, the more radioactivity. That’s why Geiger counters are so useful in hunting for uranium. Uranium is very, very radioactive. If you happen onto some uranium ore, the Geiger counter really goes wild.” “We have a Geiger counter at home,” Eddie said eagerly. “Teena and I have gone uranium prospecting several times.” “Haven’t found any uranium,” Teena said, “but we’ve had fun trying. Whoops. There’s some clicking!” She put her hands up to the earphones. Jim had moved the probe down close to the stem of the cotton plant. “Good,” he said. “We mixed a little radioactive phosphate into the ground around the roots this morning. See, the Geiger counter shows that the phosphate tracer has already started moving up into the plant. Helps show how important phosphate is to plant growth, and how eagerly the plants absorb it.” “The plant sure looks healthy enough, all right,” Eddie said. “Right,” Jim said. “Now let’s see how far up into the plant the tracer has gone.” He moved the probe upward over the smaller twigs and leaves. On the lower leaves the Geiger counter kept clicking rapidly. Eddie watched the needle stay forward on the gauge. “See, the leaves have taken a lot of it in already,” Jim explained. Then, as he moved the probe farther up toward the top of the plant, the clicking diminished until only the familiar slow background count remained. “It quit,” Teena said. “Shows that the phosphate has only reached about half of the plant so far,” Jim said. “You see, with the tracer and the Geiger counter we can tell just how far it has gone and how long it has taken. We can even tell how much has been absorbed by comparing the amount of radioactivity in the leaves and stems of the plant to what we know was contained in the original tracer.” “Boy, that’s something!” Eddie exclaimed. “By adding tracers to some fertilizers,” Jim went on, “we found that the plant made no use of the fertilizer. The Geiger counter didn’t pick up any radioactivity in the plant. Meant wasted money to any farmer or gardener who used it. Now do you see what we mean by a tracer? See how radioactive tracers can be helpful?” “Oh, yes,” Teena said. “I do.”