XIX
USING ELECTRICITY TO AID THE MEMORY
For the sparking equipment of the motor boat we use dry cells which have an internal resistance of not more than .06 ohm. They will, when short circuited through the ammeter for only an instant, give 25 amperes.
(1.5 volt)/(.06 ohm) = 25 amperes
When we allow for a slight resistance in the ammeter itself, and for the drop in voltage, we see that the internal resistance of a cell must be even less than .06 ohm.
After being used about two months upon the motor boat these cells develop more internal resistance, and they will then show not more than six to ten amperes when short circuited through an ammeter. They are then not reliable for ignition of the engine, but are quite as good as ever for bell-ringing, and often continue so for more than a year. The result is that we always have more partly run-down dry cells than we can use. Seeing them about has stimulated the boys to devise ways for using them.
The housekeeper is distracted by carrying on so many cooking processes at one time. She forgets the eggs, and lets them boil five minutes instead of three because the coffee must percolate twelve minutes, and she lets the coffee percolate twenty instead of twelve minutes because the biscuit must bake twenty minutes, and the biscuit are forgotten because the pies must come out in thirty minutes, and the cake in forty minutes. All this worries the cook. Harold is a sympathetic boy and enters into the troubles of others. I had at one time shown him how to bore a hole in a glass plate in five or ten minutes by using a round file wet with water. One day he presented the kitchen with a clock, intended to relieve the burdened memory of the cook. This is represented in [Fig. 175].
Fig. 175
An ordinary kitchen clock had a hole bored through the glass which covers its face. This glass is easily moved around in its metal rim, bringing the hole over any desired minute upon the face. One wire of the battery is attached to a leg of the clock, the other goes to a bell, and then the wire from the bell is poked through this hole. When the minute hand reaches that point the electric current is closed through the metal of the clock, and the bell rings warning that the eggs, coffee or what not are done.