The shop-door alarm for electric bells here described rings the bell during the whole time the door is open. There are two separate portions - the "contact springs" and the "separator" or " insulator." The two springs are screwed, as shown by Fig. 1, to a block of hardwood about 1 in. by 1 in. by 1/2 in., well soaked in paraffin wax. The springs may be made from an old clock spring straightened out, one end of each being filed as shown in Fig. 2, and about 1/4 in. from this end a piece of platinum foil may be soldered or riveted. The springs should be bent as in Fig. 1, so that the platinum points will be pressed well together. A binding screw and an ordinary brass screw, 1/8 in. or 5/16 in. long, should be sufficient for each spring. The " separator " is a wedge-shaped piece of hardwood, with an extension for screwing to the upper part of the door. It should be well soaked in melted paraffin wax. The contact-spring block is screwed to the lower edge of the door-frame, just above the door; and the insulator is fixed to the door, near the top, so that when the door is closed the springs are wedged apart. The alarm is next connected up to the bell and battery, one wire from a binding screw of the bell to a binding screw of the alarm, another connecting the second terminal of the alarm and one pole of the battery, and a third lead joining the free terminals of bell and battery. By the interposition of a switch in the circuit, the door can be opened by the occupants of the house or shop without the bell ringing.

Fig. 2. Shop-door Electric Alarm.