ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
597 
an intermittence in tlie current—a peculiarity which we have seen may also occur in 
tubes of lower degrees of exhaustion. But if a positive air-spark be introduced into 
the circuit which passes through the tube of high exhaust, the standard-tube will at 
once show positive effects, i.e., the positive luminosity will be severed, and the well- 
known hollow cone and highly striated termination of the truncated column will at 
once be visible. This demonstrates conclusively that the action within the tube of 
high exhaust is such as to cause charges of positive electricity to be driven from the 
tinfoil upon it in the sudden intermittent and impulsive way that is needed to produce 
the ordinary sensitive effects in a continuous current; or, in other words, it shows us 
that charges of positive electricity are rushing through the tube of high exhaust and 
affecting the tinfoil upon it. And wherever the tinfoil be placed upon the tube of high 
exhaust (unless, perhaps, in the immediate neighbourhood of the negative terminal, 
where the results may be somewhat affected by the special circumstances of the case) 
the same effects will be found to be produced. Thus, in the case of the positive air- 
spark the discharges pass through tlie tube in the shape of positive electricity. 
The above phenomena present themselves when the air-spark is in the positive, 
whenever care has been taken that neither the peculiarities of the tube nor those of 
the discharge introduce a second type of intermittence ; and to ensure their appearance 
it will generally be found sufficient to connect the negative terminal to earth, and 
to introduce an air-spark into the positive. It happens, however, not unfrequently, 
that either from the shape or size of the terminals, or their relation to the degree 
of exhaust, or from some other similar cause, a tube possesses an inherent power of 
causing an intermittence of a negative type in a discharge even though a positive air- 
spark has been introduced into it, and in such cases we, of course, get mixed results. 
And it is of great importance that this should be borne in mind, for when the resis¬ 
tance is very great the danger of results becoming mixed in their character is very 
much increased, and further complexities are doubtless produced by the escape of 
large quantities of electricity from different portions of the circuit outside the tube 
into the air. We shall have to speak of certain types of these exceptional results ; 
but in the meanwhile it is important to remember that such peculiarities as these do 
not furnish any argument against the conclusions above referred to; they should be 
looked upon as a kind of instrumental error, and they are only exaggerated forms of 
difficulties with which we have already become familiar in dealing with ordinary 
vacuum tubes.* 
Conversely, if a negative air-spark be introduced into the principal circuit the effects 
m the standard-tube will be negative in type, i.e., produced by negative impulses on 
the outer surface of the glass. If the air-spark be of a moderate size the well-known 
constriction or “ ring-terminal ” appearance will often be very clearly manifested; and 
* The only cases of such peculiarities that we have hitherto met with are those of tubes of extremely 
high vacuum, in which the negative terminal is of small size. These sometimes impart to the discharge 
a negative intermittence of tlie most violent type. 
MDCCCLXXX. 4 H 
