1876.] 
The Newly-Discovered Force . 
183 
1. The battery power may be insufficient. 
2. The eleCtro-magnet may be too large, or the core may 
be too large, or the wire may not be sufficiently fine. 
3. Sufficient pains may not have been taken to darken 
the room. The strong sparks can be seen in full light of 
gas or day, but feeble sparks can be detected only in mode- 
rate darkness. The spark of this force is always compara- 
tively weak. 
A micrometer screw is of advantage in making the adjust- 
ment of the pencil points in the dark box. In the dark box 
a spark can be seen and studied when it cannot be readily 
seen outside. Very small iron wire and rusty tools give the 
spark better than copper wire or polished metals of any 
kind, the oxide of iron giving a more brilliant spark. 
4. The person making the experiment may have connec- 
tion with the conductor, and thus draw off a part of the 
force before it reaches the dark box or other point where it 
is studied. The body is a good conductor, and error from 
this source must be constantly guarded against. 
Physical Experiments . 
Out of a very large number of other experiments, I may 
mention the following : — I stuck a penknife in a large block 
of paraffin, and connected it with metal conducting the 
force, as a gas-pipe or wire, drawing the blade lightly over 
the conductor. No sparks appeared. When a long file 
was substituted for the knife, sparks were abundant, and 
were kept up as long as the connection was made. In these 
experiments the force appears to pass into the metal, and 
thence is diffused into the air. I suspended by long pieces 
of silk rolls of wire of various sizes, and allowed them to 
strike against the connection. With small coils sparks 
rarely appeared ; with the larger coils they were abundant. 
It would seem, therefore, that a certain size is necessary in 
the conductor in order to get the sparks. A short bit of wire 
wound round a glass rod, and held against the conductor, 
would not get any spark ; but take the same bit of wire 
connected at the other end with a spool of wire, or any 
large or long metallic surface, and the spark at once appears. 
A large surface of metal seems to attract the force better 
than a small surface. For this reason it is an advantage to 
conneCt the distal pencil point in the end of the dark box 
with the gas-pipe or other large metallic conductor : it is 
not, however, necessary to do so, for the spark will appear 
when the lead pencil is isolated. At one time I led the wire 
