Abstract: The apparatus comprises first and second input terminals (1, 3) for receiving an input voltage, a number of transformers (7), each of which comprises a primary winding (9) having first (13) and second (15) ends, and a secondary winding (11). At least one end of each primary winding is connected to a switching member (17) which comprises a first controllable switching element (19) and a second controllable switching element (21). This end (15) can be connected, via the first switching element (19), to the first input terminal (1) and, via the second switching element (21), to the second input terminal (3). The switching member (17) may be in a first state or a second state, the first switching element (19) being closed and the second switching element (21) being open in the first state whereas in the second state the first switching element is open and the second switching element is closed.

Abstract: An interferometer which may be used in a Fourier transform infra red spectrometer comprises a beam splitter (10), first and second fixed mirrors (18,26), path length variation means (20), and a folding mirror (16). The optical components are mounted on a casting (8) such that the beam splitter (10) and folding mirror (16) are arranged parallel to each other and the fixed mirrors (18,26) are attached to a single face of the casting in the same plane. The path length variation means (20) comprises two parallel opposed mirrors (22,24) which are rotatable to vary the length of the optical path between the beam splitter (10) and the fixed mirror (26).