262 Prof. Oliver Lodge on the Seat of the 
usually observed when two metals are touched and separated 
is not primarily a difference of potential between the metals at 
all. They are at different potentials when separated, no doubt, 
because they are oppositely charged; but they may have been 
at the same potential until separated. The real Volta effect is 
almost independent of the true contact-force, and of the dif- 
ference of potential which it produces. In other words, a 
ood Volta effect can be observed when there was no difference 
of potential whatever between the metals when in contact. 
According to my view the Volta effect is produced, not by 
a contact-force at the junction of the two metais, but by a 
Fig. 15. 
ae 
Hydrostatic analogue of the Volta effect, or apparent difference of poten- 
tial produced by metallic contact, and of the opposite charges but 
uniform potential which it maintains between two metals in contact. 
The vessels are covered by air-tight elastic bags differently stretched. 
contact-force at their free surfaces, between the metals and 
the air or other medium surrounding them. To represent this 
hypothesis by a hydrostatic model, we shall have to maintain 
the difference of level in the two connected vessels, not by a 
force at the junction, but by a force at the surfaces ; say by 
using closed vessels and compressed air, or more pictorially by 
differently stretched elastic membranes or bladders tied over 
the tops of the vessels. 
Note that the difference of level in this case implies no dif- 
ference of potential; and, as before, no work is required to 
transfer water between A and B. Hence it is not easy to 
distinguish this case from the former; and this difficulty of 
distinguishing between the two cases is what has given rise to 
most of the confusion. The only easy criterion is the non- 
existence in the second case of any Peltier effect at the junc- 
tion C. Naturally it is possible and common for the two 
effects to be superposed, but they are essentially independent. 
Since the two vessels in the second case are at the same 
potential, the way to observe the effect is to cut and seal the 
