Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

Fig. 80. Simple Hot Wire Meter.

A simple and crude form of meter which is sufficiently sensitive for most experimental work is illustrated in Fig. 80. A piece of No. 36 B. S. platinum wire is sealed in the bulb of an ordinary air thermometer. When the wire becomes heated by a passing current of electricity, it causes the air in the bulb to expand and change the height of the colored liquid in the tube.

An air-thermometer is simply a glass tube of fine bore having a bulb blown at the upper end and the free end immersed in a reservoir of ink or some other colored liquid. The instrument is put in working order by grasping the bulb in the palm of the hand, so that the warmth of the hand will expand the air and cause some of it to escape from the lower end of the tube. Upon removing the hand, the air will contract and suck some of the liquid up into the tube. It should rise only about half way to the bulb, and the tube should be about 18 inches long so as to leave room for changes in the position of the column due to variations in the outside atmosphere. A cardboard scale graduated in inches and reading downward is fastened in back of the tube.

The tube should have a fine bore so as to make the instrument as sensitive as possible. The best liquid to use is alcohol, colored with a little aniline dye. Alcohol has a lower specific gravity than water, and the column will be more sensitive to small changes of pressure. The same figure shows a form of meter devised and used with success by the author.

Two tubes are fitted to the bulb, a large one having a bore of about 0.1 of an inch and another about 0.04 inch. Connection is established by the aid of two corks and a short length of glass tubing one inch in diameter. The tubes are bent U shaped, and a little colored alcohol is placed in each, so that the bottle reservoir is unnecessary. The tube of large bore is fitted at the top with an ordinary glass stopcock such as that used in chemical laboratories.

The stopcock is left open and the transmitter is set in operation by holding down the key. The helix, etc., are adjusted until the larger tube shows a maximum reading. The stop-cock is then closed and the instruments further adjusted by noting the reading in the finer tube which corresponds to much smaller changes in current. The finer bore cannot at first be used alone because the large changes of current would blow the liquid out of the tube. In lieu of a glass stopcock, a piece of rubber tubing may be placed over the end of the tube and closed, when necessary, with a pinch cock.

Fig. 81. Meter with Case Removed.

Fig. 81 shows a more elaborate and sensitive form of meter which is not only suitable for experimental outfits but may be used with good results for more careful work. The advantage of the form of meter here described is that it is "pivotless" so to speak, and contains no bearings which require jewels to eliminate friction.

The "hot wire" is platinum, and in order to compensate for external changes of the atmospheric temperature, is mounted on a strip of glass. Glass and platinum expand at nearly the same rate, and the wire is thus kept taut and prevented from changing the position of the pointer except when the current passes.