Draft addition to patent 833.391: thermal compensation using thermistors (26.10.1960)

In this document

This document is a handwritten draft for a request for an addition to patent no. 833.391, dated and signed by Magès: "Magès / 26.10.60" p. 91. It bears the title "Dispositif d'asservissement du moteur d'un groupe hydro-Electrique" p. 91 and the supplementary reference "(4385)" p. 91.

The problem posed

The text recalls that the main patent describes an automatic system regulating the power of a hydraulic motor driving an electric generator, so that the terminal voltage remains at a predetermined value regardless of the power demanded p. 91.

When this generator powers a storage battery, the voltage delivered must vary with the battery temperature, otherwise it would impose strong variations in charging current on it p. 91. The main patent already proposed "several thermal compensation devices"; the present addendum proposes "another solution" p. 91.

The thermistor solution

Magès explains the principle of "Thermistors", resistors with a negative temperature coefficient, used in series with the winding of an electromagnet to keep the intensity flowing through it constant despite its temperature variations (self-heating or ambient temperature) p. 91.

The chosen solution consists of inserting an additional thermistor into the electromagnet circuit, placed in close contact with the battery, "or, preferably, immersed in its electrolyte" p. 91. It is calculated to adjust the intensity delivered to the electro-hydraulic regulator's electromagnet, so that the generator voltage corresponds to that of the battery regardless of its temperature p. 91.

Pages 89 and 90 present a technical diagram repeating the reference "See figure 4 main patent (except 27)", with two thermistors marked:

• A: "Thermistance thermique de l'électro-aimant" p. 89-90
• B: "Thermistance assurant la variation de la tension en fonction de la température de la batterie" p. 89-90

The parts are numbered (1, 2, 3, 4, 18, 19), in explicit reference to figure 4 of the main patent, and connected to a block annotated "accu" p. 89-90. The reference "P 2617" appears at the bottom of both pages p. 89-90.

Context

The patent no. 833.391 mentioned belongs to the lineage of Citroën patents on hydro-electric and hydropneumatic systems developed under the direction of Paul Magès from the 1950s onwards. Requests for additions to a patent allowed, under the patent law of the time, to complete or improve a protected invention without having to file an entirely new patent.

Thermistors, electronic components whose resistance varies strongly with temperature, were at that time a common and relatively inexpensive way to compensate for thermal drifts in regulation circuits — a classic problem for any system embedded in a vehicle, subject to large variations in ambient and operating temperature.

Archivist's perspective

This short three-page file clearly illustrates Magès's working method: faced with a problem already addressed in a main patent, he documents an alternative solution, simpler, before having it protected as an addition. The presence of two almost identical diagrams p. 89-90 probably suggests a working version and a refined version of the same sketch, intended for official filing.

The use of the thermistor immersed in the battery electrolyte p. 91 testifies to Magès's constant search for direct and robust physical solutions rather than complex electronic circuits — an approach consistent with the spirit of the hydropneumatic and hydro-electric devices he was developing elsewhere, as evidenced by the related corpus on hydrostatic transmission, another major field of invention where he constantly sought to mechanically simplify the control systems.