There he set to work on a strange apparatus, wires from which ran up to a flag pole on which he was constructing what looked like a hastily improvised wireless aerial. That part arranged, Kennedy followed his wires down again and took them in by a window to a sort of lumber-room back of the office. Outside everyone was too busy to watch what we were doing there and Craig could work uninterrupted.
"What are you doing?" I asked. "Installing a wireless plant?"
"Not quite," he smiled quietly. "This is a home-made wireless photo-recording set. Of course, wireless aerials of amateurs don't hum any more since war has caused the strict censorship of all wireless. But there is no reason why one can't receive messages, even if they can't be sent by everybody.
"This is a fairly easy and inexpensive means by which automatic records can be taken. It involves no delicate instruments and the principal part of it can be made in a few hours from materials that I have in my laboratory. The basis is the capillary electrometer."
"Sounds very simple," I volunteered, trying not to be sarcastic.
"Well, here it is," he indicated, touching what looked like an ordinary soft glass tube of perhaps a quarter of an inch diameter, bent U-shaped, with one limb shorter than the other.
"It is filled nearly to the top of the shorter limb with chemically pure mercury," he went on. "On the top of it, I have poured a little twenty per cent sulphuric acid. Dipping into the acid is a small piece of capillary tube drawn out to a very fine point at the lower end."
He filled the little tube with mercury also. "The point of this," he observed, "is fine enough to prevent the mercury running through of its own weight--about as fine as a hair."
He dipped the point and held it in the sulphuric acid and blew through the capillary tube. When the mercury bubbled through the point in minute drops, he stopped blowing. It drew back for a short distance by capillary attraction and the acid followed it up.
"You can see that connections are made to the mercury in the arm and the tube by short pieces of platinum wire," he continued. "It isn't necessary to go into the theory of the instrument. But the most minute difference of potential between the two masses of mercury will cause the fine point at the junction of the liquids to move up and down.