R. C. Browne.

The herein-described instrument if carefully made will be found extremely useful in detecting minute currents of electricity, and for use in connection with the Wheatstone bridge described in the last number of this magazine. It is so named because the arrangement of the needles is such as to neutralize the earth's magnetism. It is constructed on the principle that a wire through which a current is passing, will deflect a magnetic needle towards a position at right angles with the wire.

How To Make An Astatic Galvanometer 251

Fig. 1.

A small coil of cotton or silk covered magnet wire should first be made. About one-half ounce of No. 30 gauge, white preferred, will be required, and should be wound over a block of wood 11/2" square and §" thick. The ends should be smooth to facilitate the removal of the coil when completed. The coil should be wound so that the top layer will be divided into two sections, having the same number of turns in each section. This can easily be done by driving into the block before winding, twelve strong pins, or small gauge wire nails, as shown in Fig. 1. The outer pins in the top row are 3/4" apart, the inner pins \" apart; the pins on the under side are 5/8" apart. No wire should be wound in the space between the inner pins of the top row. Wind the coils evenly and firmly by hand, leaving ends about 6" long for connections. When the winding is completed, dip the coils in hot paraffin. When cool, but not hard, remove the pins and lightly press the coila between two flat pieces of wood, to make them smooth and flat on the top and bottom. Then lay aside until the paraffin is quite hard, when the block is carefully removed.

How To Make An Astatic Galvanometer 252

Fig. 2.

The coil is mounted on a baseboard 7" square and 7/8" thick, two binding-posts being placed 1" from the corners on one side, and the ends of the coil connected to them. The coil is attached to the center of the baseboard with sealing-wax, the divided part being on top.

A dial is next made of a piece of white card-board 6" square. With dividers draw a circle 5" in diameter, then draw lines through the center at right angles, dividing it into four equal parts, as shown in Fig. 2. The ends of one of the lines mark O, the ends of the other 90°. Other divisions may be made at 15, 30, 45, 60 and 75°. Cut a slot in the center of the dial on the O line 11/4" long and 1/8" wide, and then attach it with sealing-wax to the top of the coil, so that the slot will be over the dividing space in the coil. Pieces of wood or cork may be cemented between the base and the dial, to more firmly hold the latter. Nails or iron should not be used anywhere in this instrument.

The support for the needles is made of a piece of \" brass rod 10" long. Bend 11/2" or 2" of one end to a right angle, and to the short end affix a binding-post, either by soldering or by threading the end of the rod. The hole in the binding-post should be 21/2" from the longer section of rod. Make a \" hole through the dial and into the baseboard, 21/2" from the center of the dial and on the O line, as shown in Fig. 2. Drive the long arm of the brass rod into the hole in the base, and through the hole in the binding-post on the other arm put a long, flat-headed brass nail. Place the arm so the brass nail is directly over the center of the dial.

How To Make An Astatic Galvanometer 253

Fig. 3

The needle magnets are next made. Two medium-sized sewing needles are required. Break off each end, using the center portions, which should be 1" long. Use care to get them alike in size and length. They should be magnetized by winding wire around them and sending a current from a battery through the wire for a few minutes, or by rubbing them with a permanent or electromagnet. The needles are then put through holes in a short length of fine straw, far enough apart so that when suspended they will just clear the coils, one needle being above and the other below the upper layer of the coils. In place of the straw, a stirrup of fine copper wire may be made by twisting it around the needles, the N pole of one needle being on the same end with the S pole of the other, and as evenly adjusted as possible. An index needle of fine brass wire 41/2" long should be put through the straw or stirrup, just above the upper needle, as shown in Fig. 3. The needles are suspended from the brass nail in the binding-post by means of a fine fiber of cocoon silk, or fine human hair attached to the straw or stirrup by a drop of sealing-wax, so that the lower needle will turn freely inside the coil and the upper needle just clear the dial without touching it.

How To Make An Astatic Galvanometer 254

Fig. 4.

A glass case for covering the instrument may be made by taking four pieces of window glass 10" long and 6" wide, and one piece 6" by 61/4", and cementing them together with fish glue in the form of a box with one open end (see Fig. 4). Narrow strips of cloth or leather on the edges will add to the strength.

To test the instrument, when completed, twist pieces of iron and copper wire together at one end, connecting the free ends to the binding-posts on the base. Upon holding a lighted match to the twisted joint, the electricity generated by this heat will deflect the needle. Even the heat of the hand is sufficient to produce a movement of the needle. A piece of copper in one hand and a piece of zinc in the other, applied to the binding-posts, will cause the needle to oscillate freely. In using the instrument the index needle must be adjusted by turning the brass nail so that it rests on the O line.