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http://www.google.ca/patents/US3899644?utm_source=gb-gplus-share
Patent US3899644 - AC/DC-MONITORING TELEPHONE LINE
AC/DC-MONITORING TELEPHONE LINE
Theodore R. Hunt et al
Overview
Abstract
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Description
Claims
Patent number
: 3899644
Filing date
: 19 Jun 1974
Issue date
: 1975
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