Abstract: A self-balancing DC-substitution measuring system includes a temperature dependant resistor (TDR) which is placed in one leg of a Wheatstone Bridge circuit, diagonally opposed first and second nodes of the bridge being driven respectively by the outputs of two operational amplifiers such that the potential at each of these first and second nodes, when measured with respect to a common reference point is equal in magnitude and opposite in polarity to the other. Each of the amplifiers has a pair of differential inputs and each of these inputs are connected to either of the remaining third and fourth nodes of the bridge circuit or to the common reference point, no two inputs from the same amplifier being connected to the same point and each of the third and fourth nodes and the common reference point having at least one amplifier input connected to it.

Abstract: A cold trap for use in a high vacuum system to analyze the vapor component or components of the gases in, or introduced into, the vacuum system, including the assessment of the partial pressure of any specific component which may be present in the said gases, comprising a chamber closed at its upper end by a thin metallic diaphragm dished inwardly to receive a liquid coolant such as liquid nitrogen, an outlet to interconnect the chamber with a high vacuum system, and means to introduce successive doses of liquid coolant into the diaphragm. This construction allows the trap to be rapidly cycled. It is shown to be advantageous for the radius of curvature of the lowest part of the diaphragm to be such that the area of the diaphragm which remains at the temperature of the coolant until all the coolant has evaporated is larger than the effective area of the outlet.