Page:Popular Science Monthly Volume 91.djvu/165

 Popular Science Monthly

��chloride of zinc, which is required to make up the exciting mixture. In preparing chloride of zinc, the scrap-zinc should be placed in a large stoneware crock and 3 or 4 lb. of commercial hydrochloric acid poured over it. This must be done out- doors, because the hydrogen gas that is given off is harmful to breathe, and also inflammable. A great heat is generated and the liquid may boil up and make it run over the top of the crock. A small quantity of cold water may be poured in to lessen the chemical action without detriment to the resulting product, which should be left until quite cold before using. It is very important that this solution should contain no free hydrochloric acid, because its introduction into the made-up cell would cause chemical action, and the cell would soon be destroyed. Any trace of free acid is readily dispersed by adding a small quantity of chloride of ammonium in powder form and stirring the liquid until effusing ceases. By this method a small quantity of chloride of ammonium may be introduced without harm. As soon as the solution of chloride of zinc is ready it may be strained through a piece of damp muslin to free it from dirt, paper chips and other matter, and then filtered through a tuft of cotton pressed in the neck of a glass funnel. This will free the solution of any fine iron-deposit or lead that may have been in the zinc, thus aiding the efficiency of the re-made cell. This solution should test 32 deg. Baume. Sal ammoniac, or chloride of ammonium in the crystal form, is then dissolved in the chloride of zinc. Filtered or distilled water is then added and the mixture stirred well and tested to register 32 deg. Baume. The solution is then ready for use.

The carbon-rods and the brass binding- posts and screws must be cleaned and the zinc-cylinder lined with a double thickness of blotting-paper, or blotting-board, cut so as to leave a margin of about Yi in. at the top where it is turned over on the outside. Cut some circular pieces of cardboard, or heavy blotting-board, so that they will fit tightly into the interior of the blotting paper. Press three or four of these down into the bottom of the blotting-paper lining inside of the zinc-cylinder. It is then ready for filling. Just before putting in the mixture pour in some of the chloride solu- tion so as to wet the paper all over, drain it and place the zinc upside-down to drain off the excess liquid. This takes about

��149

��20 minutes, for the paper must not be over wet. Then place upon a smooth board about 1 34 lb. of the carbon and manganese powder that has been treated. Add about 3 oz. of the chloride of zinc and chloride of ammonium solution and mix well so that it will hold together when gripped in the hand. It must not be very wet or it will not give the proper amperage. The right consistency is very important. Pour some of the carbon mixture into the paper-lined zinc-cylinder and ram it down hard ; insert one of the carbon rods, adjust it centrally, then pack some of the mixture around it and ram it down tight with a suitable strip of wood, hammering it in with a wood mallet. The carbon rod must be tapped down occasionally to prevent it from lifting.

The tighter the mixture is packed the greater will be the amperage when finished.

Cut off the outside edges of the blotting paper and fold the top of it inwards toward the carbon-rod, press it down and see that none of the carbon mixture makes contact between the zinc and the carbon-rod. This is essential as the connection would short circuit the cell. Place all the binding screws in position, test the cell with a suitable battery ammeter and see that the screws of the carbons are tightly fitted. It should give a current of 15 to 22 amperes.

Dry the sand and pack some of it over the top; then melt the tar and pour it on the sand to completely fill the cell to the top. These made-over cells will give an excellent working current for a considerable length of time.

Method of Insulating Secondary * 'Pies*' in Transformers

IN most text-books on the construction of transformers, it is customary to advise the insulating of the "pies" or sections in the secondary winding by means of long strips of empire cloth, wound over and over through the center holes of the sections. This requires a good deal of work on the part of the constructor.

While overhauling a transformer, the writer tried out a somewhat novel method. The core was insulated in the usual manner, with a number of layers of empire cloth, but the "pies," instead of being wound with strips of empire cloth, simply had round disks of empire cloth, of double thickness, between each pair of sections. This gave fully as good insulation, and made access to the different sections easier.

�� �