When a current passes through a conductor, it generates heat in proportion to the resistance offered and the amount of current flowing. Heat causes metals to expand sufficiently so that these two properties may be applied to the construction of a hot wire ammeter for the measurement of alternating currents of high frequency and potential.

The hot wire ammeter is placed in series with the aerial, so that by noting the deflection of the pointer, the inductance, capacity and spark gap may be adjusted until the meter gives a maximum reading.

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 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.

Drill four 1/8-inch holes in a piece of window glass 6 inches long and 1 inch wide. The location of the holes is shown in Fig. 82. The two at the ends serve to mount the standards, A and B, and those at the center to fasten down the strip to the base. The holes are drilled with a small three-cornered file which has been broken off and set in a breast drill. The broken end should be used to drill the glass and be kept thoroughly lubricated with camphor and turpentine. With a little care and patience the holes may be drilled without breaking the glass.

Two brass standards are fastened on each end of the glass. They are bent out of sheet brass and are 3/4 inch high and 3/8 inch wide. A brass spring of the same width and 1 1/4 inches long is clamped under one standard. The standard which holds the spring in position is tapped for a small thumbscrew which may be secured from a binding post. Solder a small brass pin to the top of the spring and another one to the top of the standard which is fastened at the opposite end of the glass strip. Some paper or rubber washers must be placed between the feet of the standards and the glass strip to prevent it from cracking when the screws are tightened.

For a station up to one-half K.W. in power the hot wire must be No. 40 B. S. gauge platinum. For larger stations a single No. 36 wire may be used or three No. 40 wires in parallel. The wire must be about 7 inches long. Stretch it between the standard, A, and the spring, C. Wrap the ends around the pins and solder them there, using as small amount of solder as possible. The tension of the wire, which should be taut, is adjusted with the thumbscrew.

Take a piece of the platinum wire about 1/2 inch long and make a little eyelet at one end. Wrap the straight end around the center of the long hot wire and tie a piece of silk in the eyelet.

The glass strip and its standards supporting the hot wire may then be fastened to the baseboard of the instrument by means of two round headed brass wood screws. Two rubber washers must be interposed between the glass and the wood. A piece of 3/32 inch brass 1/2 inch wide and 3 1/4 inches long is bent in the shape shown by F in Fig. 84. The upper end is bored and tapped to receive a thumbscrew similar to the one in the standard on the glass strip. Two brass springs 1/64 inch thick, 3/8 inch wide and 1 1/4 inches long are soldered or riveted at opposite ends of F in the positions shown in Fig. 85. The springs should project one inch from the upright. A small hook made from an ordinary pin is soldered to the outside end of each.

The movement is shown in perspective by Fig. 85. G is a rectangle of very thin copper, 1/2 inch long and 1/4 inch wide, having a little projection 1/4 inch long bent in a curve so that it forms a sextant of a circle, of which the intersection of the diagonals of G would be the center.

The pointer is a piece of steel wire 5 inches long. It is slightly flattened by hammering so that it will retain its shape and not curl. About 1/2 inch is allowed to project through G and is weighted with a lead shot so as to partly counterbalance its weight.

Two loops of wire are fastened to the corners of G by tying them in holes which are bored there for that purpose. The wire is fine phosphor bronze .003-.005 of an inch in diameter, which is used for suspending the movements of delicate galvanometers. Pass the loops over the hooks on the springs and adjust until the pointer moves horizontally. Then fasten the wires permanently to the hooks by means of a small drop of solder.

The movement is mounted in the position shown by Fig. 86. The silk thread tied to the eyelet runs to the little sextant and is cemented at the further end by means of a small drop of sealing wax. The scale is a piece of sheet copper or brass, covered with white paper and calibrated in degrees or made to read in amperes by connecting it in series with an ammeter and a source of direct current. A rheostat should be included in the circuit and the current varied so that various values may be marked off. All the different points must be located by sending an actual current of that value through the meter. An error is liable to result if any of the points are marked by guesswork, for the divisions grow smaller and smaller as they become farther away from zero. For example a position of the pointer corresponding to 0.1 of an ampere will not be half way between zero and 0.2 but will be nearer the 0.2 division.

The resistance of wires to high frequency currents is much higher than their resistance to constant currents. This would seem at first to indicate that our meter will give a higher reading for an equal current value, when used with a high frequency current after being calibrated with a direct current. But with wires of very small diameter such as No. 40 B. S. gauge there is almost no perceptible difference and consequently no error unless the frequency of the oscillations exceeds 1,000,000 per second, which is very unlikely with the "spark" method of wireless telegraphy.

Fig. 87 illustrates the form of hot wire ammeter used by the United Wireless Telegraph Co. for tuning their installations.

The pivotless meter just described should be fitted with heavy binding posts which are connected to the brass standards mounted on the glass strip by means of stranded copper wire.

The meter should be fitted with a case and glass cover to exclude dust and prevent injury to the working parts. It should be mounted in such a position that the weight of the pointer is sufficient to keep the silk thread taut so that when the wire expands the pointer which is normally at zero will fall of its own weight. When the wire cools after the current has ceased to flow, it will contract and draw the pointer up again.

Platinum wire will give good results, but for more accurate work an alloy known as platinoid is most suitable.

Detailed instructions for tuning the transmitting circuits by means of a hot wire ammeter are given in the chapter on Transmitting Helixes.