MARCH 28, 1884. | 
with its temperature. The resistance of the north- 
western and south-eastern districts of the plate will 
therefore be greater, and that of the north-eastern 
and south-western districts smaller, than before it 
was subjected to the stress; and an equipotential line 
through the centre of the plate, which would origi- 
nally have been parallel to the west and east sides, will 
now be inclined to them, being apparently rotated in 
a counter-clockwise direction. 
If the plate were of iron instead of copper, the Pel- 
tier effects would clearly be reversed, and the equipo- 
tential line would be rotated in the opposite direction. 
The peculiar thermo-electric effects of copper and 
iron, discovered by Thomson, are thus seen to be suf- 
ficient to account for Hall’s phenomenon in the case 
of those metals. It became exceedingly interesting 
to ascertain whether the above explanation admitted 
of general application; and the author therefore pro- 
ceeded to repeat Thomson’s experiments upon all the 
metals mentioned by Hall. The results are given 
in the following table, where those metals which in 
Hall’s experiments behave like gold are distinguished 
as negative, and those which behave like iron as posi- 
tive. 
| Direction 
Metals. | Forms used. of Hall’s effect. 
current. 
| 
Copper Wire and foil, pure. S. to U. Negative. 
Iron. Wire and sheet, an-; U.toS. Positive. 
nealed. 
Brass . Wire, commercial. S. to U. Negative. 
Ae . Wire and foil. WU: tos: Positive. 
Nickel . Wire. S. to U. Negative. 
Platinum . Wire and foil. S.to U. Negative. 
( Foil, purity 99.9%. Sco Wins) 
| Wire,commerc.pure.| U.toS8. | 7 
Gold 4 | Jeweller’s 18-ct. wire r Negative. 
[ and sheet. S. to U. 
Jeweller’s 15-ct.sheet.| S.to U. 
Silver . Wire and foil. S. to U. Negative. 
Aluminium . Wire and foil, pure. U. to 8. Negative? 
Cobalt . Rod, 8mm. diameter.| U.toS Positive. 
Magnesium . Ribbon. S.to U Negative. 
ein... ~.. | Foil. S. to U Negative. 
Lead . | Foil (assay). Nocurrent.| Nil. 
S. means stretched. U. means unstretched. 
It will be seen that in every case excepting that of 
aluminium, and one out of five specimens of gold, 
there is perfect correspondence between the direction 
of the thermo-electric current and the sign of Hall’s 
effect. With regard to the aluminium, a piece of the 
foil was mounted on glass, and Hall’s experiment per- 
formed with it. As was anticipated, the sign of the 
‘rotational coefficient’ was found to be positive, like 
that of iron, zinc, and cobalt. Either, therefore, Mr. 
Hall fell into some error, or the aluminium with which 
he worked differed in some respect from that used by 
the author. The anomalous specimen of gold, being 
in the form of wire, could not be submitted to the 
same test: it probably contained some disturbing im- 
purity. 
[To the foregoing article, Dr. Hall has favored the 
editor of Science with the following reply.] 
Mr. Bidwell’s table is certainly very suggestive, but 
SCIENCE. 
387 
his ‘explanation of the Hall phenomenon’ cannot 
stand. 
He makes this phenomenon to be an incidental re- 
sult of the manner in which the metal strip is attached 
to the plate of glass. It is, he says, like a beam rigid- 
ly fastened at both ends, and weighted in the middle. 
Without discussing the closeness of this analogy, one 
can see, that if we fasten the strip by its middle, and 
leave it free at both ends, the conditions upon which 
Mr. Bidwell supposes the phenomenon to depend are 
quite changed. 
After reading Mr. Bidwell’s paper, I took a strip 
of soft steel, about one-tenth of a millimetre thick, 
and made the usual connections, but, instead of fast- 
ening the strip to glass with cement, so arranged it 
that it could at will be clamped across its middle or 
across the ends to a sheet of hard rubber. The end- 
clamps were about three centimetres apart, and the 
width of the magnetic poles between which the strip 
was placed was considerably greater than three centi- 
metres. Now, when the strip was clamped across its 
middle and left free at the ends, and was made to 
conduct a current of electricity across the magnetic 
field, it was like a beam supported at the middle, and 
with a load distributed from end to end; but when 
the strip was clamped at its ends and left free in the 
middle, it was like a beam supported at both ends, 
and with a load distributed from end toend. Experi- 
ment shows that the effect is positive, as I have always 
found it in iron and steel, whether the strip be clamped 
in the middle, or at the ends. 
There is one other consideration to be urged. Mr. 
Bidwell would, I suppose, account for the fact that 
the observed effect is proportional to the magnetic 
force by saying that the strain would be proportional 
to this force. But how will he explain the fact that 
the effect is nearly or quite proportional to the cur- 
rent, as was shown in my first paper upon the sub- 
ject ? Let us see what his theory leads to. Doubling 
the current, the magnetic force remaining unchanged, 
would double the strain. But a doubled strain, with 
a doubled current, would make the heating and cool- 
ing from the Peltier effect four times as great as be- 
fore. This would deflect the equipotential lines four 
times as much as before; and, as these lines are only 
half as far apart as before, the transverse current 
would be eight times as great as before the direct 
current was doubled. The transverse effect, then, 
would be proportional, not to the current, but to the 
cube of the current. Epwin H. HALL. 
Cambridge. 
THE CREVAUX EXPEDITION. 
E. MILHOME writes from Corumba, Sept. 24, 1885, 
to the Société de géographie in regard to the possible 
survivors of the Crevaux expedition. He believes 
that several survived for a time, but were afterward 
put to death by the savages. Information of any sort 
could hardly be obtained; as the Tobas had made 
ready for war, and retired to the interior, holding no 
further communication with the neutral tribes from 
whom the previous vague news had been derived by 
the whites. The Indian survivors, from their terror 
and sufferings, can afford little help. It is known that 
Branco, one of the party, in his capacity of soldier, 
was preserved alive by the natives to instruct them 
in the use of the fire-arms which they captured; ana 
it is possible he may be still living, but, if so, has 
