  This paper explores the possibility to construct two-terminal mechanical
devices (one-ports) which are lossless and adjustable. To be lossless, the
device must be passive (i.e. not requiring a power supply) and non-dissipative.
To be adjustable, a parameter of the device should be freely variable in real
time as a control input. For the simplest lossless one ports, the spring and
inerter, the question is whether the stiffness and inertance may be varied
freely in a lossless manner. We will show that the typical laws which have been
proposed for adjustable springs and inerters are necessarily active and that it
is not straightforward to modify them to achieve losslessness, or indeed
passivity. By means of a physical construction using a lever with moveable
fulcrum we will derive device laws for adjustable springs and inerters which
satisfy a formal definition of losslessness. We further provide a construction
method which does not require a power supply for physically realisable
translational and rotary springs and inerters. The analogous questions for
lossless adjustable electrical devices are examined.
