5
RS-925D
5
Operation
Chapter 4
OpERaTIOn
4.1 Initial Inspection and Setup
This instrument was carefully inspected before ship-
ment. It should be in proper electrical and mechanical
order upon receipt.
An OPERATING GUIDE is attached to the case
of the instrument to provide ready reference to
specifications.
4.2 Connection
4.2.1 General Considerations
Four insulated low-thermal-emf binding posts labeled
CURRENT HI
,
CURRENT LO
,
SENSE HI
, and
SENSE LO
provide two current and two potential
terminals, respectively, for 4-terminal measurement.
2-terminal measurements may be made by shorting
CURRENT HI
to
SENSE HI
, and
CURRENT LO
to
SENSE LO
, preferably with shorting links or other
substantial means. The four terminal connection is
important of course for low resistances.
A fifth metal binding post labeled
GND
(Ground) is
connected to the case and may be used as a guard or
shield terminal. It may also be connected using the
shorting link to either terminal to implement a 2-ter-
minal as opposed to a 3-terminal measurement.
4.2.2 Electrical Considerations
In order to make proper use of the full performance
capabilities of the RS-925D unit, especially if low
resistance or low resistance increments are important,
care must be taken in connecting to the terminals of
the decade box.
In particular, to keep contact resistance to a mini-
mum, 4-terminal connections should be employed,
and the most substantial and secure connection to the
binding posts should be made. They accept banana
plugs, telephone tips, spade lugs, alligator clips, and
bare wire. The largest or heaviest mating connection
should be made, and, where applicable, the binding
posts should be securely tightened.
These considerations may be relaxed whenever single
milliohms not considered significant for the task be
-
ing performed.
4.2.3 Thermal emf Considerations
The highest quality low emf components are used in
the RS-925D Series. In particular, the terminals are
made of gold plated tellurium copper, which exhibits
low emf and low resistance. There nevertheless may
be some minute thermal emf generated at the user’s
test leads where they contact the RS-925D banana
jacks. This will depend on the test lead material.
Whenever this is critical, brass and iron materials
should be avoided.
This emf will not manifest itself if an ac measurement
instrument is employed. It will also be eliminated if
a meter with so called “True Ohm” capability is used.
Otherwise it may appear as a false component of the
dc resistance measurement, and can be the order of
milliohms.
