Non-Ohmic Conductors

Those conductors which do not obey Ohm's law (I V) are called non-ohmic conductors. e.g., vacuum tubes, transistors, electrolytes, etc. A non-ohmic conductor may have one or more of the following properties:

The V_I graph is non-linear i.e. V/I is variable.
The V_I graph may not pass through the origin as in case of an ohmic conductor.
A non-ohmic conductor may conduct poorly or not at all when the p.d. is reversed.

The non-linear circuit problems are generally solved by graphical methods.Fig. 15

Fig.15 illustrates the graphs of non-ohmic conductors. Note that V-I graphs for these non-ohmic conductors are not a straight line.

Thermistors

A thermistor is a heat sensitive device usually made of a semiconductor material whose resistance changes very rapidly with change of temperature. A thermistor has the following important properties :

The resistance of a thermistor changes very rapidly with change of temperature.
The temperature coefficient of a thermistor is very high.
The temperature co-efficient of a thermistor can be both positive and negative.

Construction: Thermistors are made from semiconductor oxides of iron, nickel and cobalt. They are generally in the form of beads, discs or rods (See Fig. 16). A pair of platinum leads are attached at the two ends for electrical connections. The arrangement is enclosed in a very small glass bulb and sealed. Fig.

Applications

(i) A thermistor with negative temperature co-efficient of resistance may be used to safeguard against current surges in a circuit where this could be harmful e.g. in a circuit where the heaters of the radio valves are in series as shown in fig.

A thermistor T is included in the circuit. When the supply voltage is switched on, the thermistor has a high resistance at first because it is cold. It thus limits the current to a moderate value.

As it warms up, the thermistor resistance drops appreciably and an increased current then flows through the heaters.

(ii) A thermistor with a negative temperature coefficient can be used to issue an alarm for excessive temperature of winding of motors, transformers and generators as shown in fig. Fig.

When the temperature of windings is low, the thermistor is cool and its resistance is high. Therefore, only a small current flows through the thermistor and the relay coil. When the temperature of the windings is high, the thermistor is hot and its resistance is low. Therefore, a large current flows in the relay coil to close the contacts. This completes the circuit for the signal lamp or buzzer.

(iii) Thermistors are used for voltage stabilisation, temperature control and remote sensing.

(iv) Thermistors are used to measure very low temperatures of the order of 10K.

Superconductivity

There are metals and compounds whose resistivity goes to zero below a certain temperature
T_c called critical temperature (or transition tem-perature).These materials are known as superco-nductors. This phenom-enon of zero resistance of some metals and compounds when cooled to critical temperature is called superconductivity. The critical temperature for several metals and alloys are shown in the adjoining table. At present, the problem with using the materials listed in the table for practical applications is that we must use liquid helium to cool them into the superconducting state. Liquid helium is expensive and supplies are limited.

Fig. (i) shows resistance-temperature graph for normal metals whereas as Fig. (ii) shows resistance-temperature graph for a superconductor.

Note that resistance-temperature graph for a superconductor follows that for a normal metal at temperature above T_C (critical temperature). When the temperature is at or below T_C , the resistance drops suddenly to zero as shown in fig. (ii). Recent measurements have shown that resistivities of superconductors below or less than 4 × 10^_25W m.

Material	T_C(K)	Material	T_C(K)
Al	1.20	Zn	0.85
Pb	7.19	Nb₃Ge	23.2
Hg	4.15	Nb₃Sn	18.05
Nb	9.26	NbSn₂	2.60
Sn	3.72	PbTi_0.27	6.43

One of the remarkable features of supercon-ductors is that once a current is set up in the material, the current will persist wihout any applied voltage (since R = 0). In fact, steady currents have been observed to persist in superconducting loops for several years with no apparent decay. Today there are thousands of known superconductors with critical temperatures as high as 23 K for one alloy made of a combination of niobium, aluminium and germanium. The critical temperature is sensitive to chemical composition, pressure and crystalline structure.

Applications of superconductors: The scientists are making continuous efforts to produce room temperature superconductors. Once goal is achieved, the superconductivity will offer the following uses:

(i) It will offer the possibility of loss-free transmission of electric power. It is because superconducting transmission lines would have zero resistance.

(ii) It will help for the construction of supercon-ducting magnets in which the magnetic field intesities would be about 10 times greater than those of the best normal electromagnets.

(iii) Superconducting transmission lines would permit the transmission of electric power at lower voltage, making underground power lines possible.