30 SECTIONAL ADDRESSES. 
the origin of the most far-reaching error in electrochemistry, which 
the greatest part of the scientific work in that domain has been occupied 
in fighting almost ever since. These are cited merely as representative 
specimens of the opinions of the protagonists, 
Now, there is a close connection, between thermionic emission and 
contact potential difference, and I believe that a study of thermionic 
emission is going to settle this little dispute. In fact, I rather think 
it has already settled it, but before going into that matter I would 
like to explain how it is that there is a connection between thermionic 
emission and contact potential difference, and what the nature of that 
connection is. 
Imagine a vacuous enclosure, either impervious to heat or main- 
tained at a constant temperature. Let the enclosure contain two 
different electron-emitting bodies, A and B. Let one of these, say A, 
have the power of emitting electrons faster than the other, B. Since 
they are each receiving as well as emitting electrons, A will acquire 
a positive and B a negative charge under these circumstances. Owing 
to these opposite charges A and B will now attract each other, and 
useful work can be obtained by letting them come in contact. After 
the charges on A and B have been discharged by bringing them in 
contact, let the bodies be quickly separated and moved to their original 
positions. This need involve no expenditure of work, as the charges 
arising from the electron emission will not have had time to develop. 
After the charges have had time to develop the bodies can again be 
permitted to move together under their mutual attraction, and so the 
cycle can be continued an indefinite number of times. In this way 
we have succeeded in imagining a device which will conyert all the 
heat energy from a source at a uniform temperature into useful work, 
Now, the existence of such a device would contravene the second 
law of thermodynamics. We are therefore compelled either to deny 
the principles of thermodynamics or to admit that there is some fallacy 
as to the pretended facts in the foregoing argument. We do not need 
to hesitate between these alternatives, and we need only look to see 
how the alleged behaviour of A and B will need to be modified in order 
that no useful work may appear. There are two alternatives. Hither 
A and B necessarily emit equal numbers (which may include the par- 
ticular value zero) of electrons at all temperatures, or the charges which 
develop owing to the unequal rate of emission are not discharged, even 
to the slightest degree, when the two bodies are placed in contact. 
The first alternative is definitely excluded by the experimental 
evidence, so I shall proceed to interpret the second. It means that 
bodies have natural states of electrification whereby they become 
charged to definite potential differences whose magnitudes are indepen- 
dent of their relative positions. There is an intrinsic potential difference 
between A and B which is the same, ata given temperature, whether 
they are at a distance apart or in contact. In the words of Volta, 
which I have already quoted, ‘ the metals can by themselves, and of 
their own proper virtue, excite and dislodge the electric fluid from its 
state of rest.’ 
