** A Pocket Voltammeter [330]

Remove the works and stem from a discarded dollar watch, drill two 3/16 in. holes in the edge, 3/4 in. apart, and insert two binding-posts, Fig. 1, insulating them from the case with cardboard.  Fold two strips of light cardboard, 1/2 in. wide, so as to form two oblong boxes, 1/2 in. long and 3/16 in. thick, open on the edges.  On one of these forms wind evenly the wire taken from a bell magnet to the depth of 1/8 in. and on the other wind some 20 gauge wire to the same depth.  Fasten the wire with gummed label, to keep it from unwinding.

Glue the coils to the back of the case and connect one wire from each binding-post as shown in Fig. 2, while the other two wires are connected to an induction coil lead which is inserted in the hole from which the stem was removed.  Fasten a brass-headed tack to the case at the point F with sealing wax or solder and bend a wire in the shape shown in Fig. 3 to swing freely on the tack.  Attach a piece of steel rod, 3/4 in. long, in the center coil, C, Fig. 2.

A rubber band, D, connects the steel rod C with the top of the watch case.  The ends of the rubber are fastened with sealing wax.  The rubber keeps the pointer at zero or in the middle of the scale.  Do not use too strong a rubber.  A dial may be made by cutting a piece of stiff white paper so it will fit under the crystal of the watch.  An arc is cut in the paper, as shown in Fig. 1, through which the indicator works.

To calibrate the instrument, first mark the binding-post A, which is connected to the coil of heavy wire, for amperes and the other post, V, to the coil of small wire for volts.  Connect the lead and the post marked A to one, two and three cells and each time mark the place of the pointer on the dial.  Take corresponding readings on a standard ammeter and mark the figures on the dial.  The volt side of the dial may be calibrated in the same manner, using a voltmeter instead of the ammeter.  The place where the

[Illustration:  Voltammeter in a Watch Case]

indicator comes to rest after disconnecting the current is marked zero.  —­Contributed by Edward M. Teasdale, Warren, Pa.

** A Film Washing Trough [331]

[Illustration:  Washing a Negative Film]

The washing of films without scratching them after they are developed and fixed is very difficult in hot weather.  A convenient washing trough for washing full length films is shown in the accompanying sketch.  The trough must be made for the size of the film to be washed.  Cut a 1/4-in. board as long as the film and a trifle wider than the film’s width.  Attach strips to the edges of the board to keep the water from spilling over the sides.

Cut a hole in one side of a baking powder can about half way between the top and bottom, large enough to admit a fair-sized stream of water from a faucet.  Then solder the cover to the can and punch a number of holes about 1/4 in. apart along the opposite side from where the large hole was cut.  Place this can on one end of the trough, as shown, with the large hole up.