Hawkins electrical guide ... : questions, answers & illustrations : a progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications ; a practical treatise (1914)
Hawkins, N. (Nehemiah), 1833-1928 · New York : T. Audel & Co. · leaf 316
he spring, the contact is closed and thebell is made to ring. Bells of the continuous ringing type should be used so that they willcontinue ringing when once started by the window spring irrespectiveof whether the window spring remain in the closed position or be openedby closing the window. Fig.… … Fig. 3,315.—Burglar alarm trap with string balance. The illustration shows the trap bal-anced between the contact points by the use of a string, which can be stretched in anyroom or hallway of the building to be protected from intruders. A slight disturbanceof string draws the needle to the right…
Electrical engineering
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