A
circular piece of cardboard, E, is slipped over the spring to
where the spring joins the wire. This cardboard is to serve as the
pointer. A piece of paper 1-1/2 in. wide and 2-1/2 in. long is
glued to the board so that it will be directly under the cardboard
pointer and fit snugly up against the top of the coil.
The paper can be calibrated by connecting one cell of battery to
the binding-posts. The iron plunger, D, is drawn into the tube and
consequently the pointer, E, is drawn nearer to the coil. Make a
mark directly under the place where the pointer comes to rest. At
the place mark the number of volts the cell reads when connected
with a voltmeter. Do the same with two or three cells and mark
down the result on the scale. By dividing off the space between
these marks you may be able to obtain a surprisingly correct
reading when connected with the battery cells to be tested.
--Contributed by Edward M. Teasdale, Cuba, N. Y.
** How to Make a Folding Canvas Cot [154]
All the material required to make the cot as shown in Fig. 1
consists of wood 1-1/2 in. square of which two pieces are 6 ft.
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