The invention is particularly, but not exclusively, applicable to apparatus, for example a quartz crystal microbalance system, which uses an electrical-mechanical transducer as a sensor.  In such a system, an electrical signal causes the transducer to vibrate. 
The crystal typically has an active surface on which a receptor group is immobilised.  That group has a chemical affinity or reactivity towards a substance to be detected or analysed.  The substance to be analysed is normally present in a fluid which is bought into contact with the active surface of the crystal.  Physical, chemical and biochemical interactions between the receptor group on the active surface and the substance cause a change in the mass attached to the surface (and in other physical properties of the active surface) and these affect the vibrational characteristics, in particular the resonant frequency, of the crystal.  Analysis of these effects can be used to obtain qualitative and/or quantitative data on the substance, and its interaction with the receptor. 
This type of analysing apparatus is used in situations, for example to analyse chemical and biochemical interactions, in which it may be desirable to interchange the sensor with another sensor which may be identical to the first sensor or may carry a different receptor group.  To that end, it is known to provide the sensor and a flow cell, for bringing the sample into contact with the sensor, in a cartridge which co-operates with a docking mechanism in the analysing apparatus in order to releasably connect inlet and outlet openings for the flow cell to a sample delivery/removal system.  The docking mechanism also connects the electrodes on the sensor to diving/measuring circuitry for oscillating the sensor and measuring the frequency of oscillation and/or other characteristics of the oscillation (for example the magnitude of the damping force being experienced by the sensor). 
U.S. Pat.  No.  6,196,059 shows an example of such apparatus.  The cartridge comprises an injection moulded body which has an annular rib that surrounds a surface that is to fort the base of the flow cell and that includes inlet and outlet openings which communicate with an inlet and outlet of the cartridge through small tubes.  The sensor is adhered to the rib which spaces the sensor from the other surface so that the sensor, the surface and the rib between them define the flow cell.  The cartridge is received in a docking mechanism that comprises a lid pivotally mounted on a base.  The base includes fluid conduits for connecting the inlet and outlet ribs of the cartridge to a sample delivery/removal system, whilst the lid of the mechanism includes an electrical connector for connecting the electrodes of the sensor to the drive/measurement circuitry.  Although the connector is described as being spring loaded, it appears to exert enough force on the cartridge to require that the connector engages contacts which are laterally spaced from the sensor so that the electrical connector does not exert a force directly on the sensor.  As a result, the cartridge is of a relatively complicated construction having two spaced apart contacts for the electrical connector and wires connecting those contacts to the electrodes on the sensor.