Device indicating presence of frost on road - has wheatstone bridge with air, road surface and sub-surface resistors

摘要

A wheatstone bridge monitors the air temperature immediately above the road surface and surface and sub-surface temperature. Opposed bridge branches (BCG, EDH) incorporate resistors detecting air temperature (R3) and surface (R2) with sub-surface temperature (R1) respectively, the other branches embodying fixed (52, 54) and variable (P1) resistors, and an operational amplifier (3) is connected across a diagonal (GH). The amplifier output leads to a signalling mechanism which is actuated by increments of the algebraic difference of diagonal potentials. A secondary amplifier (4) entry impedance temperature indicator (M).