“Why, perhaps eight,” said his father. “They are in a row, running along the middle of the plate. The holes are about a quarter of an inch in diameter.”
“How much is that?” asked Rollo.
“Why, I should think a common lead pencil might be a quarter of an inch in diameter. The holes are of such a size that you could just put a lead pencil in before it is cut.”
“Are the holes bored entirely through the plate?” asked Rollo’s mother.
“No,” replied his father, “a little more than half through.”
“Then,” continued he, “they have a number of little rods made of different substances,—such substances as they wish to try, in respect to their conducting power. One is made of brass, one of iron, one of glass, one of wood, &c., and these are fitted into the holes. That is, one end of each rod is made just large enough to go into its hole and take out, and yet not be loose in it.
“The upper ends of these rods are hollowed out a little, so as to form small cavities or cups, to put some phosphorus into.”
“What is that for?” asked Rollo.
“I’ll tell you,” said his father. “When they are going to use the instrument, they put all the rods into their sockets, and a little bit of phosphorus into the top of each one, in the cavity. Each piece of phosphorus is about as big as the head of a pin. Now, when these rods are all arranged in this way, the whole apparatus is put upon some hot place, where the whole plate will be heated alike, as, for example, upon the top of a stove. The heat from the stove is conducted into the iron plate; from the iron plate it is conducted into the ends of the rods which are in the sockets; thence it is conducted up the rods towards the phosphorus at the tops of them, only it goes up much more slowly through some than through others. And we can tell which rod conducts the heat the most rapidly, for the phosphorus upon the top of that rod will be the first to take fire.”
“Will the phosphorus take fire?” said Rollo.