This is a circuit of a catalytic gas detector with a 4-20mA output the light represents the heating element which resides with is the sensing element resister when no gas is present the resistance reads 500 ohms when cal gas is applied (50% LEL) the gas will touch the bead element egniting the gas transfering the energy to the element increasing the temperature thus increasing resistance to 600 ohms which will increase output current to 20 mA. This is regarded as resistive bridge circuit and the principle is used in many instruments
