Variable resistances

The.se arc similar to stator resistance starters, as discussed in Section 4.2..3 and can be used in the rotor circuit to control the rotor side resistance. Figure 5.10 shows the smooth variation of resistance by electrolytic vaporization compared to a conventional metallic resistance variation. I hc self-variable resistance of electrolyte is equivalent to almost three or four steps of a metallic resistance and makes such starters economical. Normally one step is sufficient for motors up to 160 h.p. (For spccd-torquc... 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

Consider now the transfer of electrons from electrode II to electrode I by means of an external source of e.m.f. and a variable resistance (Fig.. 20b). Prior to this transfer the electrodes are both at equilibrium, and the equilibrium potentials of the metal/solution interfaces will therefore be the same, i.e. Ey — Ell = E, where E, is the reversible or equilibrium potential. When transfer of electrons at a slow rate is made to take place by means of the external e.m.f., the equilibrium is disturbed and Uie rat of the charge transfer processes become unequal. At electrode I, /ai.i > - ai.i. 3nd there is... 

Galvanostatic polarisation—constant direct current power units, or banks of accumulators or dry cells used in conjunction with a variable resistance. 

The essential requirements for a constant-current electrolytic determination — a source of direct current (which may be a mains-operated unit producing a rectified smoothed output of 3-15 volts), a variable resistance, an ammeter (reading up to 10 amperes), a voltmeter (10-15 volts), and a pair of platinum electrodes — can be readily assembled in most laboratories, but if a number of determinations are to be performed a commercial electrolysis unit will doubtless be preferred. This will be equipped with rectifier, a motor drive for a paddle-type stirrer or with a magnetic stirrer, and a hotplate. 

Ey and E2 are the indicator electrodes. These may consist of a tungsten pair for a biamperometric end point for an amperometric end point they may both be of platinum foil or one can be platinum and the other a saturated calomel reference electrode. The voltage impressed upon the indicator electrodes is supplied by battery B (ca 1.5 volts) via a variable resistance Rs N records the indicator current. For a potentiometric end point Ey and E2 may consist of either platinum-tungsten bimetallic electrodes, or Ey may be an S.C.E. and E2... 

The simple electrical circuit shown in Fig. 16.15(h) is suitable for this procedure. The voltage applied to the titration cell is supplied by two 1.5 V dry cells and is controlled by the potential divider R (a 50-100 ohm variable resistance) it can be measured on the digital voltmeter V. The current flowing is read on the micro-ammeter M. 

An electrical testing load system was prepared as shown below using two variable resistance potentiometers rated at 0 to 1.0 ohm at 25 watts, a current measuring shunt, and two digital multimeters. 

Variable resistance (C) adjusts the intensity of current, whereas inductance coil (B) stabilizes its flow. 

Figure 4.5 A potentiometer circuit The voltage from the test circuit is balanced against a known voltage by means of a variable resistance using a galvanometer to indicate the position at which no current flows in either direction.

With fixed resistances, Rx and R2, the variable resistance Rv is adjusted until no current flows through the galvanometer. Under these conditions ... 

It is well known, for example, that about 5 per cent of the people who become infected with typhoid germs become typhoid carriers and continue indefinitely to harbor an active colony (harmless to them) of typhoid bacilli in their intestinal tracts. There is abundant evidence of graded susceptibility there are many mild cases, and the incubation time in different individuals may be from 3 to 30 days, indicating variable resistance. The fact that about 70 per cent of the carriers are women suggests that the constitutional characteristics which make it possible for individuals to endure the presence of the organisms are in some way sex-linked in their inheritance. The evidence with respect to typhoid carriers, diphtheria carriers, scarlet-fever carriers, poliomyelitis-virus carriers, etc., makes it seem probable that "carriers" enter into the dissemination of many other milder infections, and... 

Solvents Good behaviour with aliphatic hydrocarbons Attacked by aromatic hydrocarbons, chlorinated solvents, esters, ethers and ketones Variable resistance to oils, greases, alcohols... 

The cell voltage E and current density j were recorded using a high-power potentiostat (Wenking Model HP 88) interfaced with a variable resistance in order to fix the current applied to the cell and with a PC to apply constant current sequences and to store the data. 

The sensing filament in the gauge head forms a branch of a Wheatstone bridge. In the TFIERMOTRON thermal conductivity gauges with variable resistance which were commonly used in the past, the sensing filament was heated with a constant current. As gas pressure increases, the temperature of the filament decreases because of the greater thermal... 

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