817,258. Temperature alarm systems. SPEED DEVELOPMENT CO. Ltd. Oct. 10, 1955 [Oct. 18, 1954], No. 29989/54. Class 118(1). A temperature alarm apparatus for aircraft engines and so on, operating at rapid temperature rise, has shrouded thermocouples 5b and exposed couples 5a, the resultant voltage from which at alarm condition energizes the coil 7 of a switch M<SP>1</SP> causing the moving contact needle 2 to make with a contact 6 and energize the winding 8 of a relay L<SP>5</SP>, which relay closes self-holding contacts 12, 13; warning lamp contacts 21, 22; and contacts 15, 16 which short out the relay L5 when the winding 14 of a relay L<SP>4</SP> is energized on return to normal conditions and movement of the contact 2 back to contact 3. The winding 8 of relay L<SP>5</SP> is energized from a 24-volt source S through a resistor R<SP>3</SP> having a 16-volt drop, contacts 2, 6 and leads 10, 11 to earth return and S, this circuit including change over contacts 9a which are rapidly opened and closed by self-shorting relays L<SP>1</SP>, L<SP>2</SP> to introduce a pulsating supply. The holding circuit for the winding 8 comprises lead 9, resistor R<SP>3</SP>, contacts 12, 13, lead 12a, winding 8 and return lead 11. At the moment of contact between the contacts 2 and 6 the operating coil 7 is supplied with energy from the thermocouplies and from the source S in series with the couples, so that the energy holding the contacts 2 and 6 together is increased. The voltage from the source S is also applied to assist the break of contacts 2 and 6 when they are so operated. As soon as the holding circuit operates for the winding 8, however, the current from the source S is diverted from the coil 7 through the contacts 12, 13, the coil 7 then being energized only by the thermocouples. If normal temperature condition is resumed the coil 7 is de-energized and the contact 2 returns to the contact 3, and in so doing energizes the winding 14 of relay L<SP>4</SP>, the circuit for the winding 14 being as follows : Source S, lead 9, contacts 56, 57 of a test relay L<SP>7</SP>, winding 14, contacts 3, 2, resistor R<SP>1</SP>, lead 9b, contacts 19, 20, leads 9c and 9x, contacts 9a, leads 9y and 13a, contacts 13, 12 and the winding 8 of relay L<SP>5</SP>, and thence to the negative 11 of source S. The relay L<SP>4</SP> then operates and shorts the winding 8 of relay L<SP>5</SP> through contacts 15, 16. The shorting of L<SP>5</SP> breaks the contacts 12, 13 which de-energize the winding 14 and releases the relay L<SP>4</SP>. Test buttons 18 are provided to energize the relay L<SP>7</SP> and break contacts 56, 57 to cut out the release relay L4. At the same time contact 19 is changed over to contact 58 which releases the short across the resistor R<SP>2</SP> connected in parallel with the thermocouples. This introduces across the thermocouple circuit a voltage equal to their warning output, since the point r<SP>2</SP> of the resistor R<SP>2</SP> is connected to the lead 11 and source S through contacts 19, 58, and through R<SP>2</SP>, contacts 9a and resistor R<SP>3</SP> to the positive side of S. The coil 7 is thus test energized. The apparatus may be applied to each engine of, say, a four-engined aircraft, Fig. 2 (not shown), in which case the pulsating relay and test button systems are slightly modified and additional fuses are provided. Specification 687,136 is referred to.