you will soon
forget.

APPARATUS 15.

_26. Battery Fluid; Bichromate Solution._ For running small motors,
shocking coils, etc., this solution will be found good when used with
the zinc and carbon elements given in App. 3 and 4. The bichromate
destroys the hydrogen bubbles which help to polarize cells so rapidly
when the plain dilute acid (App. 14) is used. (Study polarization.) The
zinc used in this fluid must be well amalgamated (App. 20).

Directions. With 1 quart of cold water placed in a glass or earthen
dish, slowly mix 4 fluid ounces of commercial sulphuric acid. Read
Sec. 22 carefully. When this gets about cold, add 4 ounces of
bichromate of potash. Powdered bichromate will dissolve more quickly
than the lump. Keep this fluid in corked bottles, labelled, with date:

Bichromate Battery Fluid.
Apparatus 15.

27. Always take the zinc from this fluid as soon as you have finished
experimenting, or even if you have no use for the cell for a few
minutes. The zinc and fluid are rapidly destroyed in bichromate cells
even when the circuit is open. Always wash the carbon and zinc as soon
as you take them from the fluid.

APPARATUS 16.

_28. Battery Fluid._ For 2-fluid cells (App. 7), a saturated solution of
copper sulphate (blue vitriol) is needed. Place some of the crystals in
a glass jar, with water, stir them around, and add the sulphate as long
as it is dissolved. A few extra crystals should be left in the stock
bottle so that the solution will always be saturated.

APPARATUS 17.

_29. Vinegar Battery Fluid._ For a few of the experiments with
detectors, etc., good strong vinegar does well as the exciting fluid.
This may be used with the copper and zinc or carbon and zinc elements.
The amount of current given with vinegar and App. 4 or 5 is sufficient
to show many of the simpler experiments.

APPARATUS 18.

_30. Battery Fluid._ Strong brine, made by dissolving ordinary salt in
water, will produce quite a little current with App. 4 or 5. The
presence of the current is easily shown with the astatic detectors.

APPARATUS 19.

_31. Measures for Water, Acids, etc._ If you do not own a graduated
glass, such as druggists use for measuring liquids, the following plan
will be found useful. In the mixing of battery fluids, etc., while it is
not necessary to be absolutely exact, it is necessary to know
approximately what you are doing.

An ordinary glass pint fruit jar may be taken as the standard. Th