Balance force-coil

Force-Coil Analytical Balances. The most precise modern analytical balances use a "force coil," mounted in a magnetic field the current needed to counterbalance the weight placed is measured (electromagnetic force restoration balance) or an optical system measures displacement these can be sensitive to 1 part in 3 x 106. Some "window comparators" yield a precision of 1 part in 109. 

Active Sensors The heart of the active sensor is a device measuring the ground acceleration, the so-called force balanced accelerometer (FBA) (Fig. 3). The FBA has a feedback coil, which can exert a force equal and opposite to the inertia force due to the acceleration. The displacement transducer sends a current to this force coil... 

Since these forces work in opposition, the actual coil dimensions describe the point of balance between the two. 

The quantity b has the dimension of a volume and is known as the excluded volume or the binary cluster integral. The mean force potential is a function of temperature (principally as a result of the soft interactions). For a given solvent or mixture of solvents, there exists a temperature (called the 0-temperature or Te) where the solvent is just poor enough so that the polymer feels an effective repulsion toward the solvent molecules and yet, good enough to balance the expansion of the coil caused by the excluded volume of the polymer chain. Under this condition of perfect balance, all the binary cluster integrals are equal to zero and the chain behaves like an ideal chain. 

The behavior of a bead-spring chain immersed in a flowing solvent could be envisioned as the following under the influence of hydrodynamic drag forces (fH), each bead tends to move differently and to distort the equilibrium distance. It is pulled back, however, by the entropic need of the molecule to retain its coiled shape, represented by the restoring forces (fs) and materialized by the spring in the model. The random bombardment of the solvent molecules on the polymer beads is taken into account by time smoothed Brownian forces (fB). Finally inertial forces (f1) are introduced into the forces balance equation by the bead mass (m) times the acceleration ( ) of one bead relative to the others ... 

Figure 5.1 Example TG design. As the specimen changes weight, its tendency to rise or fall is detected by the LVDT (see section 7.3). A current through the coil on the counterbalance side exerts a force on the magnetic core which acts to return the balance pan to a null position. The current required to maintain this position is considered proportional to the mass change of the specimen.

Current balance — Historic instrument to measure currents by measuring the repulsive force between metal wire coils through which currents flow. The electromagnetic force is balanced by gravity acting on a known mass. Such instruments have been used for the absolute determination of the ampere (-> SI base units). 

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