_P´´_ is a push button situated upon the wall by the side of the door which leads from the dining-room to the kitchen. When the circuit is closed at this point, the current leaves the battery at _c_, passes up to the right-hand binding post of the kitchen bell and divides, part of it going through each bell. The portion which goes through the kitchen bell passes around its magnets and through its armature to the left-hand binding post, then up to the middle binding post of the upper bell, through its armature to its left-hand binding post and back to the battery by way of the push button _P´´_. The other portion of the current passes directly up to the right-hand binding post of the upper bell, around its magnets, and through its armature to its left-hand binding post, thence back to the battery by way of the push button _P´´_. Hence, both bells clatter and keep time with each other. The upper bell will ring independently of the lower bell, but the lower bell is dependent upon the upper one to open and close its circuit, somewhat as a relay. Soon after the cottage had been equipped with electric bells I went to the mill one day and found a push button at the door. Upon going in I was curious to examine the electric bell outfit of that place and found what is illustrated in Fig. 174. [Illustration: Fig. 174] A switch, _S_, had been attached to the bell. The boys said that when they felt well they kept the switch upon the left-hand point and the bell rang as a clatter bell. When they felt a little sick they put the switch upon the middle point and the bell rang with a single stroke, but when they felt very sick they put the switch upon the dead point and the bell did not ring at all. 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.