the test is made.

_Potential of Test Thimbles._ It has been found an easy matter to so
arrange the contacts in the jacks of a multiple switchboard that
whenever the line is idle the test thimbles of that line will be a
certain potential, the same as that of all the unused calling plug tips.
It has also been easy to so arrange these contacts that the insertion of
a plug into any one of the jacks will, by virtue of the contacts
established, change the potential of all the test thimbles of that line
so that they will be at a different potential from that of the tips of
the calling plugs. It has not been so easy, however, to provide that
these conditions shall exist under all conditions of practice. A great
many busy tests that looked well on paper have been found faulty in
practice. As is always the case in such instances, this has been true
because the people who considered the scheme on paper did not foresee
all of the conditions that would arise in practice. Many busy-test
systems will operate properly while everything connected with the
switchboard and the lines served by it remains in proper order. But no
such condition as this can be depended on in practice. Switchboards, no
matter how perfectly made and no matter with how great care they may be
installed and maintained, will get out of order. Telephone lines will
become grounded or short-circuited or crossed or opened. Such
conditions, in a faulty busy-test system, may result in a line that is
really idle presenting a busy test, and thus barring the subscriber on
that line from receiving calls from other lines just as completely as if
his line were broken. On the other hand, faulty conditions either in the
switchboard or in the line may make a line that is really busy, test
idle, and thus result in the confusion of having two or more subscribers
connected to the same line at the same time.

_Busy-Test Faults._ To show how elusive some of the faults of a busy
test may be, when considered on paper, it has come within the
observation of the writers that a new busy-test system was thought well
enough of by a group of experienced engineers to warrant its
installation in a group of very large multiple switchboards, the cost of
which amounted to hundreds of thousands of dollars, and yet when so
installed it developed that a single short-circuited cord in a position
would make the test inoperative on all the cords of that
position--obviously an intolerable condition. Luc