Improved Mechanical and Physical Indicating and Regulating Apparatus.
Year: 1889

page_text: Date of Application, 22nd Feb., 1889
Complete Specification Left, 21st Nov., 1889-Accepted, 8th Feb., 1890
IKUVISIONAL SPECIFICATION.
Improved Mechanical and Physical Indicating and Regulating
Apparatus.
WILLIAM EDWARD AYRTON, Professor of Physics, of 2 Upper Phillimore Gardens
London, W. and JOHN PERRY Professor of Engineering of 10 Penywern Road
London S.W. do bereby declare the nature of this invention to be as follows:-
A metallic rod, wire, or strip whose cross section is such that two dimensions at
right angles to one another are very different, a very narrow rectangle or elongated
ellipse, for example, we shall call a strip. We have found that such a strip after we
have given to it a twist about its axis so that it retains its twisted shape acts in much
the same way as our Magnifying Spring described in our paper read before the Royal
Society and published in the Proceedings No. 230 of 1884 and otherwise well known
as one of our Patented inventions. The twisted strip can be used in any of the ways
in which our magnifying spring is used, or for any other measurement of forces, or
the magnification of motion.
We sometimes make our springs or strips so that one part is right handed and the
remaining portion left handed in its twist or spiral, and if a pointer be fixed at the
neutral point and one of the ends gets an axial motion of translation merely, turning
being prevented there the pointer at the middle turns round.
Now we find when such a spring or strip which we here call a double spring or strip
is fixed at its euds there being a certain initial tension, if it is prevented from altering
in length, the pointer rotates, when the temperature of the spring or strip alters.
Hence in this form it can be used as a thermometer, or if the alteration of temperature
is due to passing a current of electricity through the spring or strip it may be called
an Electric Current Measurer and may be used in Ammeters or Voltmeters or for
many kinds of regulation in Electrical Appliances where the alteration of a current is
employed to produce the regulation.
Sometimes instead of a double spring or strip we use a single strip A B and a wire
of the same material B C fastened to the end of A B at B, where there is a pointer,
the ends A and C being rigidly fixed. There is no alteration of the length A B or of
BC when A B and B C are heated to the same temperature, but if there was an initial
tension in A B C the pointer rotates.
Sometimes we use a single spring or strip fast at one end A and pivoted at the other
end B to which a pointer is attached. The distance from A to B is kept constant, an
initial tension existing in the spring, and when the temperature of the spring alters;
the pointer rotates.
The instrument in all cases may be used to measure change of temperature or to
measure any agency which produces a change of temperature, as an electric current.
We sometimes use two single springs or strips, fastened near enough to one another
at, the ends to be free to turn without touching, in place of one such spring or strip,
particularly for electric current measurement, the current flowing along one A. B, and
hack by the other B C, the ends B and B being in metallic contact, and the
ends A and C being insulated from one another.
In electrical measuring instruments, especially in those depending for their action
on the heating of a conductor, and therefore in which the current density is large, its
is desirable, in order to prevent the instrument from being damaged, should the current
exceed its normal value, to provide a fuse made of such a material, and of such a
sectional area, that it is melted by a current which is slightly in excess of the
maximum working current used with the instrument, but which is less than the
current which will damage the instrument.. In the case of instruments of high
resistance, such as Voltmeters required to measure many volts, the introduction
of such a fuse does not seriously increase its resistance or diminish the sensibility, but
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with instruments of low resistance, such as Voltmeters to measure only one volt, the
introduction of even a short fuse that will melt with a current lower than what will
damage the instrument materially increases the resistance of the instrument, and so
diminishes its sensibility. To evercome this defect we employ the following device:-
The fuse is made of such a section that it would require far more current to fuse it
than would be necessary to damage the instrument, and so would by itself afford no
protection if merely attached to the instrument; but in addition we arrange that
when the pointer, or some other moving part of the instrument, is deflected by the
passage of the current through any pre-arranged distance beyond that through which
it is deflected in the ordinary working of the instrument, a contact is automatically
made which has the effect of short-circuiting the instrument and causing the current
to pass only through the fuse and the contact pieces and the fuse now melts.
Dated this Twenty-first day of February 1889.
WILLIAM EDWARD AYRTON.
JOHN PERRY.
COMPLETE SPECIFICATION.
Improved Mechanical and Physical Indicating and Regulating
Apparatus.
WILLIAM EDWARD AYRTON Professor of Physics of 2 Upper Phillimore Gardens,
London W. and JOHN PERRY Professor of Engineering of 10 Penywern Road
London S.W. do hereby declare the nature of this invention and in what manner the
same is to be performed, to be particularly described and ascertained in and by the
following statement:-
A metallic rod, wire, or strip whose cross section is such that two dimensions at
right angles to one another are very different, a very narrow rectangle or elongated
ellipse, for example, we shall call a strip. We have found that such a strip after we
have given to it a twist about its axis so that it retains its twisted shape, whether this
retention of shape is by a permanent set or by means of an auxiliary spring or in
other ways, acts in much the same way as our magnifying spring, now well known
through many publications but first described in our Patent Specification No. 2156
of 1883. The twisted strip can be used in any of the ways in which our magnifying
spring is used or for any other measurement of forces or the magnification of
motion.
As we find it difficult to give verbally a description of the nature of the twist, we
shew in fig. 1 a side view of a portion of a twisted strip or nearly rectangular
section.
We sometimes make our springs or strips so that one part is right handed & the
remaining portion left handed in its twist or spiral, and if a pointer be fixed at the
neutral point and one of the ends gets an axial motion of translation merely, turning
being prevented there, the pointer at the middle turns round.
Now we find that when such a strip, which we here call a double strip, is fixed at
its ends there being a certain initial tension, if it be prevented from altering in length,
the pointer rotates when the temperature of the strip alters. Hence in this form it
can be used as a thermometer; or if the alteration of temperature is due to passing
a current of electricity through the strip it may be called an electric current measurer,
and may be used in Ammeters & Voltmeters, or for any kind of regulation in
Electrical appliances where the alteration of a current is employed to produce the
regulation.
Of course, if the middle of the double strip is held fast, the ends will rotate. It is
a case of relative motion.
In carrying out our invention it must be remembered that the behaviour of the
strip is due to its change of temperature, & also to the change of temperature of the
case, as this alters the length of the strip. If the mere rise of temperature of the strip
itself is to be measured, the strip must be kept of constant length or as nearly constant

page_text: length as possible. If the difference of temperature between the strip and its case is
to be measured, and this is what we have to deal with in the above mentioned
ammeters, voltmeters, and other electrical appliances, then the axial coefficient of
expansion of the case must be made as nearly as possible the same as that of the
strip.
Sometimes instead of a double strip we use a single strip A B and a wire of
the same material B C fastened to the end of A B at B, where the pointer is placed,
the ends A & C being rigidly fixed. There being no alteration of tue length A B or
of BC when A B and 15 C are heated to the same temperature, if there is an initial
tension in A B C the pointer rotates as the rise of temperature takes place.
Sometimes we use a single strip, fast at one end A, & pivoted at the other end B
to which the pointer is attached. The distance from A to B is kept constant,
an initial tension existing in the strip, and when the temperature is altered the pointer
rotates.
When a straight strip is taken, one end A fixed, the other end B twisted, so that
there is the same kind of twist everywhere, & the end B is held in this position, then
whether much or only a little permanent set exists, when a portion of this strip alters
in temperature & not the other, we find that more of the twist comes into the
portion which is at the higher temperature, & a pointer at the point of junction may
be used to indicate the effect. Thus if an electric current be allowed to heat only a
portion of such a strip we have a means of measuring it.
We sometimes use two single strips fastened near enough to one another at the ends
to be free to turn without touching, in place of one such strip, particularly for electric
current measurement, the current flowing along one A B and back by the other B¹ C,
the ends B & B being in electrical contact and the ends A & C being insulated from
one another.
The instrument in all cases may be used to measure change of temperature or to
measure any agency which produces a change of temperature, as an electric current.
Thus as above mentioned, we sometimes pass current through the strip. But for the
measurement of high potential differences it would be advisable to use a fine carbon
strip, & in this case we find that the fixing of the ends, the strip being under pulling
or tensile stress, is very troublesome. If, however, the metallic strip merely acts as a
thermometer, the heat being electrically generated in a spiral of thin carbon
surrounding the metallic strip, this spiral need not be subjected to any considerable
stresses & the instrument may be used for electric measurement or regulation. Again,
for the measurement of large currents, a metallic, or other conducting tube
surrounding or nearly surrounding the metallic strip may have the heat electrically
generated in it, the strip merely acting as a thermometer.
In electrical measuring instruments especially in those depending for their action on
the heating of the conductor, and therefore in which the current density is large, it is
desirable, in order to prevent the instrument being damaged should the current exceed
its normal value, to provide a fuse made of such a material & of such a sectional area,
that it is melted by a current which is slightly in excess of the maximum working
current used with the instrument, but which is less than the current which will damage
the instrument. In the case of an instrument of high resistance, such as a voltmeter
required to measure many volts, the introduction of such a fuse does not seriously
increase its resistance or diminish the sensibility; but with an instrument of low
resistance such as a voltmeter to measure only one volt, the introduction of even a
short fuse, that will melt with a current lower than that which will damage the
instrument, materially increases the resistance of the instrument & so diminishes its
sensibility. To overcome this defect we employ the following device:-
The fuse is made of such a section that it would require far more current to melt it
than would be necessary to damage the instrument & so would by itself afford no
protection if merely attached to the instrument; but in addition we arrange that when
the pointer or some other moving part of the instrument is deflected by the passage
of the current through any prearranged distance beyond that through which it is
deflected in the ordinary working of the instrument, a contact is automatically made

page_text: which has the effect of short circuiting the instrument & causing the current to pass
only through the fuse & the contact pieces, & the fuse now melts.
For the purpose of making our meaning quite clear we will describe the carrying
out of our invention in one important case: a twisted strip voltmeter with the fuse
arrangement for protecting it.
The instrument is shewn in fig. 2. A B C is a double twisted strip of Platinum-
Silver fastened at the ends A & C to the ends of an iron brass frame D, which is
tubular but which allows the pointer F fixed to the strip at B to rotate nearly
through four right angles, so that the part of the frame E is a thin but rigid bracket
connecting two tubular parts. The frame is two thirds of brass in its axial length
& one third of iron so as to compensate for change of temperature of the case, so that
there may be no rotation of the pointer if the case & strip are altered equally in
temperature. One end C of the strip is insulated from the case so that current may
be passed through the strip. At the end A there is a screw for adjusting the initial
pull. We find that when a definite difference of potentials is maintained between the
ends of the strip the pointer points to a definite part of the scale placed below it, so
that this scale may be graduated in volts. Or again, it may be graduated in
amperes. The construction of the rest of the instrument is easily understood from
the figure.
The fuse arrangement & the electrical connections are shewn in fig. 3. A B C is
the double strip; T & t are the terminals of the instrument. Current from T passes
through the fuse M & through the fine wire NPN to A B C & t.. The wire NP N,
which we generally make of platinum silver because of its permanent elastic properties
at high temperatures, & because of its small changes of resistance, is kept tight by
means of the spring Q, attached to it at the middle point 1 by means of a piece of
glass or other insulating substance, & the point P moves as the current increases
because the wire N P N expands. Now should the current become so great as to be
dangerous for the strip A C, N P N yields so much that the spring Q makes contact
with the screw R and consequently the instrument is short circuited, a large current
passing through the fuse M through Q & R to t, & the fuse melts. The contact
Screw R is adjusted so that contact is made when the current is just a little larger
than the maximum current that passes through the instrument in its ordinary,
working. In the instrument we have a slate base K & the brass blocks m, m, n, n, r,
are fixed to this, & the whole covered over with a brass box L (fig. 2). The resistance
of the fuse M is so small that small changes in it do not effect the accuracy of the
instrument.
Having now particularly described and ascertained the nature of our said inven-
tion, and in what manner the same is to be performed, we declare that what we
claim is:
1. The use of a tightened twisted strip for the measurement of temperature in
the way described.
2. The use of a tightened twisted strip for the measurement or indication of electric
currents or regulation by electric currents in the way described.
3. The use of a tightened twisted strip, which as described twists or untwists
with change of temperature, to measure or indicate such physical effects as produce
change of temperature.
4. The use for the protection of electrical measuring instruments of the fuse
arrangement described, whereby when some part of the instrument moves over a
prearranged distance a contact is made & the instrument short circuited through a
fuse.
Dat this 21st day of November 1880