A system of physical chemistry - Index of


A system of physical chemistry - Index of


had already suggested that more than one type of ionisation is to be expected.

At the present time there is no general agreement as to the

actual significance of each of these types.

We now turn to the Selective Photo- Electric Effect.

Elster and Geitel in 1894 discovered that in certain cases the photoelectric

effect depended upon the plane

light. The substance examined by

them was the liquid alloy of sodium

and potassium. It was found that

when light fell upon the surface at

an angle of 45°, a maximum photoelectric

current was produced when

of polarisation of the incident

the electric vector of the light was

vibrating in the plane of incidence.

(Compare, however, the conclusions

arrived at by Millikan, infra,')

Further, in this so-called selective

effect the number of electrons

emitted has a maximum value for

a particular frequency of the light.

This maximum value occurs at a

wave-length somewhat shorter than

Fig. II.—The dotted line indicates the

normal effect ; the maximum, the

selective effect.

the threshold wave-length Xq (for the normal effect) and is not to be

confused with A.,, itself. For wave-lengths shorter than that correspond-

falls off. The

ing to the maximum emission the selective effect rapidly

relative magnitude of the normal and selective effects are shown in the

above diagram (Fig. 11).

The position of the maximum in the selective effect occurs at the

wave-lengths given in the following table (quoted from Hughes' Photoelectricity)


Maximum Selective


Effect Occurs at

Rb 470-510 MM

K 440

Na 340



Ba . . . . , 280

The same effect is observed when the metals are in the colloidal

state in photo-electric cells. Thus the maximum sensitivity of such

cells occurs at the following wave-lengths :—

Rb 480 fx/j.

K 440

Na 320

Cs 550

No general agreement has yet been arrived at regarding the relation

of the selective to the normal effect. As indicating the obscure nature

of the selective effect, it may be mentioned that Wiedmann {Ber. d.

Deutsch. phys. Gesell., 18, 333 (19 16)) has found that the presence of

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