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_ Feb. £5, 1883 | 
NATURE 
377 
A short movable carbon rod was placed across and at right 
angles to a similar rod which was fixed in a horizontal position, 
and arrangements were made for varying and accurately mea- 
suring the pressure of the one upon the other, for varying and mea- 
suring the current passing through them, and for measuring the 
resistance at the points of contact. The following are the most 
important results arrived at :— 
1, Carbon Contacts.—Changes of pressure produce propor- 
tionately greater changes of resistance with small pressures than 
with great pressures, Thus, when the pressure was increased 
from *25 to °5 grms., the resistance fell from 16’I to 11‘o ohms., 
the difference being 5°1 ohms ; whereas, when the pressure was 
increased from 25 to 50 grms., the resistance fell from 2‘r to 
1°8 ohms., the difference being only *3 ohms. 
Changes of pressure produce proportionately greater changes 
of resistance with weak currents than with strong currents. Thus 
when the pressure was increased from ‘25 to °5 grm., the resist- 
ance fell from 9°27 to 8°45 ohms with a current of 1 ampere; 
and from 25°50 to 17°75 ohms with a current of ‘oor ampere. 
Changes of current, the pressure remaining constant, produce 
greater changes of resistance with small currents than with large 
currents, and with light pressures than with heavy pressures. 
When the resistance of a carbon contact has been reduced by 
an increase of pressure, it will, on the removal of the added 
pressure, rise to approximately its original value. 
The passage of a current whose strength does not exceed a 
certain limit, depending upon the pressure, causes a permanent 
diminution in the resistance (so long, of course, as the contacts 
are undisturbed), and the stronger the current, the greater will 
be such diminution, 
When the strength of the current exceeds a certain limit, the 
resistance is greatly and permanently increased (generally becom- 
ing infinite). The greater the pressure, the higher will be such 
limit. 
Unless special means are adopted for maintaining a constant 
current, the fall in the resistance which attends increased pres- 
sure is greater than that which is due to increased pressure alone 
being partly due also to the increased current. 
It is not proved that the diminished resistance which follows 
an increase of current is an effect of tenyperature. 
2. Metallic Contacts.—For the sake of comparison, a few ex- 
periments were made with metals. The metal principally used 
was bismuth, which was selected on account of its high specific 
resistance, but experiments were also made with capper and 
platinum. 
In the case of bismuth, and probably of other metals :—With 
a given pressure, the weaker the current the higher will be the 
resistance. This effect is most marked when the current is 
small. Thus, with a pressure of ‘1 grm, the resistance, with a 
current of ‘I ampere, was 2 ohms; with ‘ol ampere it was 
16°92 ohms ; and with a current of ‘oor ampere it was 143°3 
ohms. With a pressure of *5 grm., the resistance with the same 
currents as before was 1°45, 1°47, and 3°8 ohms. 
The passage of a current, even when very small, causes a 
permanent adhesion between metallic contacts. This effect had 
been previously observed by Mr. Stroh. 
Increase in the current is accompanied by a fall of resistance, 
and if the current be again reduced to its original strength, the 
resulting change in the resistance will be small, and it will in no 
case return to its original value. 
Diminution in the strength of the current is followed by a 
small fall in the resistance if the metal is clean, and by a small 
rise in the resistance if the metal is not clean. 
Increased pressure produces a greater fall in the resistance 
with small pressures than with great pressures, and with weak 
currents than with strong currents. 
The resistance, after having been reduced by increased pres- 
sure, does not return to its original value when the added pressure 
is remoyed. 
3. Reasons for the Superiority of Carbon over Metal in the 
Microphone.—The above observations may perhaps furnish an 
answer to the question, Why does carbon give far better results 
than any metal when used inthe microphone? The mere fact 
that a current causes delicately-adjusted metal contacts to adhere 
to each other seems sufficient to account for the superior efficiency 
of carbon, In addition to this phenomenon of adhesion, and 
probably connected with it, are the facts that metallic contacts, 
unlike those of carbon, do not even approximately recover their 
original resistance when once it has been reduced by increased 
pressure or increased current, unless indeed complete separation 
occurs ; and even the initial effect of pressure upon resistance is 
in general much more marked with carbon than with metals. 
Lastly, it is to be noticed that in the case of carbon, pressure 
and current act in consonance with each other: pressure diminishes 
the resistance, and in so doing, increases the strength of the 
current ; and the current thus strengthened effects a further 
diminution in the resistance. In the case of metals, on the other 
hand (or at least in the case of clean bismuth) pressure and cur- 
rent tend to produce opposite effects. The resistance is dimi- 
nished by pressure, and the current consequently strengthened ; 
but by reason of the increased strength of current, the resistance 
is higher than it would have been if the current had remained 
unchanged. The effect of this antagonism is not very great, but 
it seems sufficient to give a material advantage to carbon. 
The paper contains fifteen tables, four curves, and three 
diagrams, illustrative of the apparatus used. 
February 8.—‘‘ Note on Terrestrial Radiation.” 
Tyndall, F.R.S. 
On Hind Head, a fine moorland plateau about three miles 
from Haslemere, with an elevation of 900 feet above the sea, I 
have recently erected a small iron hut, which forms, not only a 
place of rest, but an extremely suitable station for meteorological 
observations. Here, since the beginning of last November, I 
have continued to record from time to time the temperature of 
the earth’s surface as compared with that of the air above the 
surface. My object was to apply, if possible, the results which 
my experiments had established regarding the action of aqueous 
vapour upon radiant heat. 
Two stout poles about 6 feet high were firmly fixed in the 
earth 8 feet asunder. From one pole to the other was stretched 
a string, from the centre of which the air thermometer was sus- 
pended. Its bulb was 4 feet above the earth. The surface 
thermometer was placed upon a layer of cotton wool, on a spot 
cleared of heather, which thickly covered the rest of the ground. 
The outlook from the thermometers was free and extensive ; with 
the exception of the iron hut just referred to, there was no house 
near, the hut being about 50 yards distant from the thermometers. 
On November 11, at 5.45 p.m., these were placed in position, 
and observed from time to time afterwards. Here are the 
By John 
results :— 
6) pm; Air 36° Fahr. Wool 26° Fahr. 
8.10 ” te ” 36 ene » 25 
9:15 3, +» 9) 30 os ny 25 
air almost dead calm, sky clear, and stars shining. 
November 12, the wind had veered to the east, and was rather 
strong. The thermometers, exposed at 5 p.m., yielded the fol- 
lowing results :— 
5.15 p.m. Air 38° Wool 33” 
5-45 ” tee ” 38 wee i oth 
6.45 ” oe ” 38 ee » 35 
9 ” wee » 39 sieja ” 36 
During the first and last of these observations the sky was 
entirely overcast, during the other two a few stars were dimly 
visible, 
On November 13, 25, and 26, observations were also made, 
but they presented nothing remarkable. 
It was otherwise, however, on December ro, On the morning 
of that day the temperature was very low, snow a foot deep 
covered the heather, while there was a very light movement of 
the air from the north-egst. Assuming aqueous vapour to play 
the part that I have ascribed to it, the conditions were exactly 
such as would entitle us on @ frtori grounds to expect a con- 
siderable waste of the earth’s heat. At 8.5 a.m. the thermo- 
meters were placed in position, having left the hut at a common 
temperature of 35°. The cotton wool on which the surface 
thern.ometer was laid was of the same temperature. A single 
minute’s exposure sufficed to establish a difference of 5° between 
the two thermometers. The following observations were then 
made :— 
8.10 a.m. Air 29° Wool 16° 
8.15 ,, 9» 29 pes poe 
Thus, in ten minutes, a difference of no less than 17° had 
established itself between the two thermometers. 
Up to this time the sun was invisible: a dense dark cloud, 
» In April, 1882, the author communicated this observation to Mr. Preece, 
who referred to it in a paper read at the Southampton meeting of the Brit. 
Assoc., on ‘“ Recent Progress in Telephony,” 
