Wire length meter suppling current to a wire from which a signal representative of length is derived

An apparatus for determining the length of wires includes connector assemblies, a current source circuit, a voltage gain circuit, analog-to-digital converter, a read-only memory, a microprocessor and a display unit. The connector assemblies are connected across the ends of the wire whose length is to be determined. The current source circuit is used to selectively supply different values of currents to the wire whose length is to be determined. The voltage gain circuit is responsive to the different values of currents for selectively generating a plurality of different voltage levels for each of the different values of currents so as to provide an analog voltage signal having a predetermined range. The analog-to-digital converter is responsive to the analog voltage signal for converting the same to a first digital signal corresponding to a resistance of the wire. The read-only memory is used to store data representative of resistance per unit length of various sizes of wires having a uniform cross-sectional area and for storing programmed instructions. The microprocessor utilizes the first digital signal corresponding to the resistance of the wire and the stored data representative of resistance per unit area for generating a second digital signal representative of the length of the wire. The display unit is responsive to the second digital signal for indicating the wire length.

BACKGROUND OF THE INVENTION 
This invention relates generally to electrical measurement devices and more 
particularly, it relates to a method and apparatus for determining the 
length of wires wound on a reel and the like on an effective and efficient 
basis. 
As is well known, wires or cables are typically wound on a core or reel for 
storage and use. These wires or cables may be stranded or solid which vary 
in diameter from 24 AWG to 1000 MCM and in length from 15 feet to 20,000 
feet. In use, certain lengths of wire are unreeled from the core and then 
cut as needed. One of the difficulties encountered heretofore by consumers 
of such wires or cables is to quickly determine their remaining 
inventories of these various wires. One way is to physically unwind each 
of the cores of wires or cables and measure the length thereof. Even if 
this was possible in each instance, it would be quite difficult and 
expensive as well as time-consuming, thereby increasing substantially the 
labor costs. 
It would therefore be desirable to provide a wire length meter like that of 
the present invention which can measure quickly and display the wire 
length of a wide range of wires while they are still wound on a reel. The 
apparatus for determining the length of wires of the present invention is 
a portable unit which can be hand carried, has its own power supply 
source, and can be readily operated without requiring special skills on 
the part of the user. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
method and an apparatus for determining the length of wires wound on a 
reel which is relatively simple and economical to manufacture and 
assemble. 
It is another object of the present invention to provide an apparatus for 
determining the length of wires wound on a reel which is of a rugged 
construction and can be operated without any special skills on the part of 
the user. 
It is another object of the present invention to provide a wire length 
meter which includes a selectable current source circuit, a selectable 
voltage gain circuit, an analog-to-digital converter, a microprocessor, a 
read-only memory and a display unit for measuring quickly and displaying 
the length of a wire. 
It is still another object of the present invention to provide a wire 
length meter which includes a temperature compensation network to provide 
automatic adjustment for ambient temperatures, thereby increasing its 
accuracy and reliability. 
It is yet still another object of the present invention to provide a wire 
length meter which includes a calibration mode of operation so as to 
enable measurement of non-standard gauge wires. 
In accordance with these aims and objectives, the present invention is 
concerned with the provision of a method and an apparatus for determining 
the length of wires in which the apparatus includes connector assemblies 
for connecting across the ends of the wire whose length is to be 
determined, a current source circuit, a voltage gain circuit, an 
analog-to-digital converter, a read-only memory, a digital processor, and 
a display unit. The current source circuit is used to selectively supply 
different values of currents through the wire whose length is to be 
determined. The voltage gain circuit is responsive to the different values 
of currents and selectively generates a plurality of different voltage 
levels for each of the different values of currents so as to provide an 
analog voltage signal having a predetermined range. The analog-to-digital 
converter is responsive to the analog voltage signal for converting the 
same to a first digital signal corresponding to a resistance of the wire. 
The read-only memory is used to store data representative of resistance per 
unit area of various sizes of wire having a uniform cross-sectional area 
and to store programmed instructions. The digital processor is responsive 
to the program instructions stored in the read-only memory for controlling 
the operation of the current source circuit, the voltage gain circuit, and 
the analog-to-digital converter so as to generate the first digital signal 
corresponding to the resistance of the wire. The digital processor 
utilizes the digital signal corresponding to the resistance of the wire 
and the data representative of the resistance per unit length for 
generating a second digital signal representative of the length of the 
wire. The display unit is responsive to the second digital signal for 
indicating the wire length.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now in detail to the drawings, there is shown in FIG. 1 a 
simplified block diagram of an apparatus 10 for determining the length of 
wires or cables, constructed in accordance with the present invention. The 
apparatus 10 is a wire length meter and includes a digital input/output 
(I/O) circuit portion 12, a selectable current source circuit portion 14, 
a selectable voltage gain circuit portion 16, a keyboard portion 18, and a 
display unit or portion 20. A reel of wire or cable sample 22 whose length 
is to be determined has its ends 22a, 22b thereof connected via respective 
connector assemblies 23a, 23b to the current source circuit portion 14. 
The current source circuit portion 14, voltage gain circuit portion 16, 
and display portion are controlled by the microprocessor-based digital I/O 
circuit portion 12 which directs specific values of current from the 
current source circuit portion 14 to be passed through the wire sample 22. 
With the current source circuit portion 14 selectively supplying different 
values of current through the wire whose length is to be determined, the 
voltage gain circuit portion 16 is used to measure the voltage across the 
ends of the wire so as to selectively generate a plurality of different 
voltage levels for each of the different values of current. 
Since both the current passing through the wire sample and the voltage 
across the ends of the wire sample are known, it is easy to establish an 
equation based upon Ohm's Law for determining the value of the resistance 
R of the wire: 
EQU R=V/I (1) 
where: 
V is the measured voltage across the ends of the wire sample 
I is the current passed through the wire sample. 
It is also generally known that the resistance R of a conductive material 
can be expressed mathematically as follows: 
##EQU1## 
where: L is the length of the conductive material 
A is the cross-sectional area of the material 
.sigma. is the conductance of the material. 
By inserting equation (2) into equation (1) and solving for the length L , 
there is given: 
##EQU2## 
where: .rho.=1/.sigma.=resistivity. 
Further, it is known that resistivity is a function of temperatures of the 
material and can be expressed mathematically as follows: 
EQU .rho.=.rho..sub.20 [1+.alpha.(t.sub.A -20)] (4) 
where: 
.rho..sub.20 is the resistivity of the material at 20.degree. C. 
.alpha. is the temperature coefficient at 20.degree. C. 
t.sub.A is the ambient temperature. 
Finally, by substituting equation (4) into the above equation (3) we have: 
##EQU3## 
where: the bracketed fraction is equal to the reciprocal of resistivity 
per cross-sectional area. 
The digital I/O circuit portion 12 includes a microprocessor and a 
read-only memory for storing data representative of the resistivity per 
unit length of various sizes of wires having a uniform cross-sectional 
area and for storing programmed instructions. Based upon the stored 
information, measured voltage and supplied current, the microprocessor can 
calculate the length of the wire utilizing equation (5) above and the 
length can then be indicated on the display portion 20. 
To more fully understand the procedure used to determine the length of the 
wires in the present invention, reference is directed to FIG. 2 which is a 
pictorial representation of a front panel 24 of the apparatus 10 where an 
operator interfaces to the digital I/O circuit portion 12 of FIG. 1. The 
front panel 24 is used to house the keyboard portion 18 and the display 
portion 20. The panel 24 further includes an on-off power switch 26; six 
light-emitting diodes (LED) 28, 30, 32, 34, 36, 38; a first connector 
socket 40; and a second connector socket 42. The first socket 40 receives 
a plug-in section of the first alligator clip connector assembly 23, the 
alligator end thereof being attachable to one end of the wire to be 
tested. Similarly, the second connector socket 42 receives a plug-in 
section of the second alligator clip connector assembly 23b, the alligator 
clip end thereof being attachable to the other end of the wire to be 
tested. The power switch 26 is switched to the "on" position for supplying 
battery power sources to the apparatus 10. Thus, the apparatus 10 is 
self-contained and needs no external source of electrical power. 
The keyboard portion 18 of the front panel 24 includes a first row of 
push-button switches 48, 50, 52, 54 which are labeled "GAGE", "1", "2", 
and "3", respectively. The front panel also includes a second row of 
push-button switches 56, 58, 60, 62 which are labeled "AUGHT", "4", "5", 
and "6", respectively. Further, the front panel includes a third row of 
push-button switches 64, 66, 68, 70 which are labeled "MCM", "7", "8", and 
"9", respectively. Finally, the front panel includes a fourth row of 
push-button switches 72, 74, 76, 78 which are labeled "CALIBRATION", 
"COPPER/ALUMINUM", "0", and "LENGTH (FEET)", respectively. 
The LED 28 or 30 will be lit initially when the power switch 26 has been 
moved to the "on" position so as to indicate the type of wire to be 
tested. Let's assume that the LED 28 is lit so as to indicate the a 
"copper" wire is being tested. If the push-button switch 74 is depressed 
once, the LED 28 will become extinguished and the LED 30 will be lit 
indicating that the type of wire to be tested is "aluminum." By depressing 
the push-button switch 74 again, the LED 30 will become extinguished and 
the LED 28 will be lit again indicating that a "copper" wire is being 
tested. 
The mode of operation is selected by depressing one of the push-button 
switches 48, 56, 64 or 72. Depending upon the uniform cross-sectional area 
of the wire or cable to be tested, which is of a standard gauge, one of 
the switches 48, 50 or 64 is depressed and the corresponding LED 32, 34 or 
36 will become lit so as to indicate the kind of wire size. The operator 
then attaches the alligator clip ends of the connector assemblies 23a, 23b 
to the respective ends 22a, 22b of the wire 22 to be tested. Next, the 
push-button switch 78 labeled "LENGTH (FEET)" is depressed. As a result, 
the display portion 20 will indicate the length of the wire. When the wire 
or cable to be tested is of a non-standard wire gauge, the push-button 
switch 72 labeled "CALIBRATION" is depressed so as to place the apparatus 
10 in the calibration mode, which enables the determination of the length 
of any gauge wire as will be explained more fully hereinafter. 
In FIGS. 3(a)-3(c), when connected together, there is illustrated a 
detailed schematic circuit diagram of the apparatus for determining the 
length of wires of FIG. 1. Specifically, in FIG. 3(a) there is shown the 
digital I/O control circuit portion 12 which is comprised basically of a 
microprocessor 80 and a read-only memory (ROM) 82 which is used to control 
the microcomprocessor. The microprocessor 80 is preferably of the type 
similar to the one commercially available from Zilog of Sunnyvale, Calif., 
as Model No. Z8681. The ROM 82 is preferably of the type commercially 
available from Motorola Corporation of Schaumburg, Ill., as Model No. 
2732. The ROM 82 is used for storing data representative of the 
resistivity per unit length of various sizes of wires having a uniform 
cross-sectional area as well as for storing programmed instructions for 
controlling the microprocessor 80. The microprocessor 80 inputs all the 
needed data and outputs all needed control signals to accomplish the 
measurement operation as set forth herein. 
The inputs to the microprocessor 80 is provided by an operator through the 
12-key keyboard portion 18 which is connected to a key-pad encoder 84 via 
eight lines designated X1, . . . X4 and Y1, . . . Y4. The key-pad encoder 
84 translates the data on these eight lines into a 4-bit hexadecimal 
signal (KP1, KP2, KP3, KP4) on respective output lines 86, 88, 90, 92. The 
encoder 84 further generates a keypad ready signal KPR on output line 94. 
The microprocessor 80 is used to poll the ready signal KPR and then reads 
in the hexadecimal signal KP1, . . . KP4 to determine which one of the 
keys was depressed. From these inputs, the microprocessor 80 generates 
control signals LED.0., LED1 and LED2 on respective lines 81, 83, 85 which 
are decoded by a decoder 96. The signals LED.0. on the line 81 is decoded 
so as to cause the turning on of one of the two LEDs 28, 30 to indicate 
whether the apparatus 10 is measuring a "copper" or "aluminum" wire. The 
signals LED1 and LED2 on corresponding lines 83, 85 are decoded so as to 
turn on one of the four LEDs 32, 34, 36 or 38 to indicate whether the 
apparatus is in the "GAGE", "AUGHT", "MCM", or "CALIBRATION" mode of 
operation. 
The microprocessor 80 provides further control signals through outputs on 
multiplexed address/data bus AD.0.-AD7, address bus A8-A15, a data run 
line DARUN, a LCD data line LCDATA, and a LCD clock line LCDCLK. The 
control signals on the address/data bus AD.0.-AD7 are received by latches 
100, 102, 104 and an analog-to-digital converter (A/D) 106. The outputs of 
the latch 100 communicates with the read-only memory ROM 82. The outputs 
of the latch 102 are used to control the voltage gain circuit portion 16 
(FIG. 3(b)) via connections at nodes GB1, . . . GB8. The outputs of the 
latch 104 are used to control the current source circuit portion 14 (FIG. 
3(c)) via connection nodes CB1, . . . CB4. The A/D converter 106 receives 
an input from the voltage gain circuit portion 16 via connection at node 
GBOUT and generates an output data ready signal DARDY which is fed to the 
microprocessor 80. A latch 108 receives the address bus A8-A15 and sends 
the address bus A8-A11 to the ROM 82. The latch 108 also sends the address 
bus A13-A15 to a decoder 110. The decoder 110 is used to decode the 
address bus A13-A15 from the output of latch 108 and generates output 
signals ROMEN, GAINEN, CURREN, ADLBEN, and ADHBEN. These output signals 
route address and data to the memory mapped elements 82, 102, 104 and 106. 
The control signals LCDATA and LCDCLK are used to drive a liquid crystal 
display of the display portion 20, which is formed of a 7-segment display 
having five digits, via LCD driver chip 112. A RC circuit 114 is connected 
to the driver chip 112 for generating a specific oscillator frequency. The 
driver chip 112 provides thirty-three parallel independent lines, 
designated by a bus 116, which corresponds to the 7-segments of each of 
the five digits so as to form the alpha-numerical and decimal point on the 
liquid crystal display. 
The selectable voltage gain circuit portion 16 of FIG. 1 is shown in detail 
in FIG. 3(b). The voltage gain circuit portion 16 includes a precision 
voltage divider chain 118, a divider chain selector chip 120, a fixed gain 
chopper stabilized amplifier 122, a low-offset amplifier 124, an ambient 
temperature compensation network 1216, a microprocessor interface circuit 
128, and a voltage regulator circuit 130. 
The voltage divider chain 118 is formed of a series-connection of four 
resistors 132, 134, 136 and 138. The resistor 132 has its one end 
connected to an input node 140 for receiving a positive terminal of a 
first power supply 188 (FIG. 3(c)) and has its other end connected to one 
end of the resistor 134 at a node A. The input node 140 is also connected 
to the end 22a of the wire 22 to be tested. The other end of the resistor 
134 is connected to one end of the resistor 136 at a node B. The other end 
of the resistor 136 is connected to one end of the resistor 138 at a node 
C. The other end of the resistor 138 is connected to an input node 142 
which is also connected to a ground potential. The input node 142 is also 
connected to the other end 22b of the wire to be tested. The voltage cross 
the ends of the wire to be tested can be measured or sensed across the 
input nodes 140 and 142 as will be more fully described. The measured 
voltage across the input nodes 140 and 142 is divided by 10 at the node A, 
divided by 100 at the node B, and divided by 1,000 at the node C. 
The amplifier 122 is formed of an operational amplifier 123, an input 
resistor R1, a feedback resistor R2, a potentiometer R3, and a resistor 
R4. The operational amplifier 123 is configured as a non-inverting 
amplifier with a fixed gain of X200 and provides a maximum output of 2 
volts. Thus, the input voltage to the op amp 123 must be scaled down to 10 
mV. This scaling is done by the divider chain selector chip 120 which is 
controlled by the microprocessor 80 via the connections at the nodes GB1, 
. . . GB4. As a result, the divider ratio at either nodes A, B or C is 
selected by the microprocessor in order to meet the 10 mV input criteria 
of the operational amplifier. The output of the divider chain selector 
chip 120 is fed to the input of the op amp 123 via a low pass filter 144 
formed of a resistor R5 and capacitor C1. 
The amplifier 124 includes an operational amplifier 125, an input resistor 
R6, a feedback resistor R7, a potentiometer R8, a resistor R9, a 
potentiometer R10, a resistor R11, and a resistor R12. The op amp 125 is 
also configured as a non-inverting amplifier with a selectable gain of 
X10, X20, or unity gain as controlled by a selector chip 129. When the 
potentiometer R8 and the resistor R9 are used, the output of the op amp 
125 provides a gain of X10. When the potentiometer R10 and the resistor 
R11 are used, the output of the op amp 125 provides a gain of X20. 
Finally, when the resistor R12 is used, the output of the op amp 125 has a 
unity gain. 
The microprocessor interface circuit 128 is formed of a level translator 
circuit which is used to interface between the digital control signals at 
the node connections GB5 . . . GB8 from the latch 102 and the selector 
chip 129. The translator circuit 128 includes a plurality of transistors 
Q1 . . . Q4 whose bases receive the respective digital control signals 
from the latch 102. Each of the diodes D1 is connected to the collector of 
a corresponding transistor so as to insure proper turnoff due to the 
baseemitter voltage drop variations of the transistors and the variations 
of the digital level of the control signals from the latch 102. 
The ambient temperature compensation network 126 is a conventional 
temperature-to-voltage integrated circuit which is commercially available 
and provides a 5 mV/.degree.C. output signal. This output signal is fed 
into the chip selector 129 so as to provide a temperature compensating 
voltage to be delivered into the op amp 125, as selected by the 
microprocessor, for amplification either to 50 pk mV/.degree.C. or 100 
mV/.degree.C. The output of the op amp 125 provides an analog voltage 
signal having a predetermined range and is representative of the 
resistance of the wire being tested. This analog voltage is sent from the 
node connection GBOUT to the input of the A/D converter 106 (FIG. 3a)). 
The voltage regulator circuit 130 is a DC/DC converter which includes a 
first battery power source 146, an op amp 148, and a voltage regulator 
150. The circuit 130 converts the battery source 146, which is typically 
+9 VDC, to a regulated +5 volts and -5 volts for use by all of the 
circuits except for the ones in the current source circuit 14. 
The current source circuit 14 of FIG. 1 is illustrated in detail in FIG. 
3(c). The current source circuit 14 includes four current loop sections 
152a, 152b, 152c, 152d which are used to selectively pass specific 
different values of current from end 22a of the wire 22 to be tested to 
the other end 22b thereof. The current source circuit 14 further includes 
a reference generator 154 and four divider chains 155a, 155b, 155c, 155d. 
The reference generator 154 is formed of an op amp 156; diodes D2, D3; a 
potentiometer 158; and a resistors 160. A stable reference voltage of +2.5 
volts is generated at the output of the op amp 156. Each of the divider 
chains 155a-155d include a first resistor 162, a potentiometer 164, and a 
second resistor 166. One end of the resistors 162 is connected to the 
reference voltage of +2.5 volts and the other end of the resistor 162 is 
connected to one side of the potentiometer 164. The other side of the 
potentiometer 164 is connected to one end of the resistor 166. The other 
end of the resistor 166 is connected to the ground potential. The outputs 
of the divider chains are at the wiper arms of the potentiometers 164 
which are set to be approximately 0.6 volts. 
The current loop sections 152a includes an operational amplifier 168a, a 
voltage-to-current converter 170a, a feedback op amp 172a, and an 
opto-isolator 174a. The op amp 168a has its non-inverting input connected 
to the wiper arm of the potentiometer 164 of the divider chain 155a for 
receiving the 0.6 volts, its inverting input connected to the output of 
the feedback op amp 172a via a resistor 176a. The output of the op amp 
168a is also connected to its inverting input via a capacitor 178a. 
The voltage-to-current converter 170a is formed of a transistor Q5a, a base 
resistor 180a, and an emitter feedback resistor 182a. The opto-isolator 
174a includes a light-emitting diode 184a and a photo-transistor 186a. The 
light-emitting diode 184a has its anode connected to the node connection 
CB1 for receiving a control signal from the latch 104 and its cathode 
connected to the ground potential. The photo-transistor 186a has its 
collector connected to the output of the op amp 168a and its emitter 
connected to the base resistor 180a. When the control signal at the node 
CB1 is activated, the light-emitting diode will conduct so as to turn on 
the photo-transistor 186a. As a result, the output of the op amp 168a will 
be coupled to the base of the transistor Q5a. The transistor Q5a has its 
collector connected to the input node 142 (the wire end 22b) and its 
emitter connected to one end of the emitter resistor 182a and to the 
non-inverting input of the feedback op amp 172a. The other end of the 
emitter resistor 182a is connected to the ground potential. 
The current loop sections 152b, 152c and 152d have identical components and 
their interconnections are the same as just described with respect to the 
current loop section 152a and thus will not be repeated. As can be seen, 
like components in current loop sections 152b, 152c, 152d have been 
designated with the same reference numerals as the current loop section 
152a followed by the corresponding letter "b", "c", or "d." It should be 
noted that the actual value of the emitter resistors 182a . . . 182d are 
different. In particular, a 0.6 volts will be developed across the 
corresponding emitter resistors 182a, 182b, 182c, 182d when a 
corresponding current of 0.001, 0.01, 0.1 and 1 ampere flows therethrough. 
A second battery power source 188, which is typically +6 volts, is used to 
supply the different currents through the wire sample. As previously 
pointed out, the positive terminal of the second power source 188 is 
connected to the input node 140 (the wire end 22a) for supplying the 
current. The opto-isolators 174a . . . 174d serve to isolate the second 
power source 188 from the first power source 146 which is to operate all 
of the other circuits in the apparatus 10. 
For completeness in the disclosure of the above-described apparatus, but 
not for purposes of limitation, the following representative values and 
component identifications used in the circuitry of FIG. 3(a)-3(c) are 
submitted. These values and components were employed in an apparatus that 
was constructed and tested and which provides high quality performance. 
Those skilled in this art will recognize that many alternative elements 
and values may be employed in constructing apparatuses and circuits in 
accordance with the present invention. 
______________________________________ 
T TYPE OR VALUE 
______________________________________ 
MP 80 8681, Zilog 
ROM 82 2732, Motorola 
Latch 100, 102, 104, 108 
74373, Motorola 
LCD Driver 112 145453, Motorola 
Encoder 84 74922, Motorola 
Decoder 96 47139 
Decoder 110 74138 
A/D converter 106 ICL7109 
Selector Chip 120, 129 
14066 
Temp.compensation network 129 
Op Amp 123, 125 741 
Transistors Q1-Q4 2N3906 
Diodes D1, D2, D3 IN914 
Op Amp 168a-d, 172a-d 
LM324 
Transistors Q5a-Q5d 
Opto-isolators 174a-174d 
Voltage regulator 78L05 
______________________________________ 
FIGS. 3(a)-3(c) and FIG. 4 will now be referenced to more fully describe 
the operation of the microprocessor 80 during a test measurement of a wire 
or cable. FIG. 4 is a detailed flow chart of the program stored in the 
microprocessor 80 and in the ROM 82 and how it is interfaced with the 
keyboard portion 18 and the display portion 20 so as to operate the 
apparatus 10. Initially, the power switch is turned on so as to actative 
the keypad shown in block A10. Then the operator selects the "GAGE", 
"AUGHT", "MCM", or "CALIBRATION" mode of operation by depressing one of 
the push-button switches 24, 56, 64 or 72. The microprocessor 80 will 
generate the control signals from block A12 to light up one of the 
corresponding LEDs in block A14 indicating the mode of operation selected. 
Next, the operator selects either the "ALUMINUM" or "COPPER" mode by 
depressing the switch 76. The microprocessor will then generate a control 
signal from block A16 to light up one of the LEDs in block A18 indicating 
the type of wire selected. One of the alligator clip assemblies 23a is 
connected to one end 22a of the wire 22 to be measured, and the other one 
of the alligator clip assemblies 23b is connected to the other end 22b of 
the wire 22. 
The operator will then depress the push-button 78 labeled "LENGTH (FEET)". 
The microprocessor will initially check for an error for the mode limit in 
block A20. If there is an error, it will be displayed from block A22. If 
no error exists, the microprocessor will begin processing under the 
control of the stored program in the ROM 82. In block A24, the 
microprocessor will selectively supply different values of currents from 
the current source circuit portion 14 and selectively measure the voltages 
from the voltage gain circuit portion 16 so as to obtain a voltage drop 
across the wire sample 22 to be within the range of the A/D converter 106. 
In blocks A26, A28, A30 the voltage and offset are sampled and then a 
resistance of the wire is calculated. 
If the "GAGE", "AUGHT", or "MCM" mode has been selected, the calculated 
resistance in block A30 is temperature compensated for in block A32. Data 
representative of resistivity per unit length of various sizes of wires 
having a uniform cross-sectional area is stored in the ROM 82. In block 
A34, the appropriate value of resistivity per unit length is retrieved 
from the ROM 82 for the type of gauge wire selected. In block A36, the 
calculated resistance of the wire in block A30 is divided by the value 
obtained in block A34 so as to calculate the length in feet. If the 
"COPPER" mode is selected, this value is displayed in block A40. If the 
"ALUMINUM" mode is selected, the value in block A36 is divided by a 
constant in block A38 prior to being displayed in the block A40. 
On the other hand, if the "CALIBRATION" mode is selected, the resistivity 
per unit length of the unknown sample wire must first be computed since 
the cross-sectional area of the wire is not known. Thus, the resistivity 
per unit length has not been previously stored in the ROM 82. Accordingly, 
when the switch 72 is depressed, the display portion 20 will show a 
reading of "CSL" which refers to Calibrate Sample Length. A specific 
length of the unknown sample wire must be cut from the reel, whose ends 
are connected to the clip assemblies. The operator must then use the 
push-button switches labeled "0" to "9" so as to enter the corresponding 
specific length of the unknown wire sample from 10 feet to 100 feet. The 
number entered will be displayed via blocks A41 and A43. Then, the 
microprocessor in block A42 will divide the resistance of the wire 
calculated from the block A30 by the length entered in block A44 so as to 
compute the resistance per foot. The microprocessor will cause this value 
to be stored in block A46. The entire reel of the unknown wire is next 
connected to the clip assemblies and the total resistance is obtained from 
the block A30 and is divided by the stored value in the block A44. This is 
calculated in block A46 and finally, the length is displayed in the block 
A48. 
From the foregoing detailed description, it can thus be seen that the 
present invention provides a method and apparatus for determining the 
length of a wire wound on the reel on an effective and efficient basis. 
The wire length meter of the present invention is formed of a selectable 
current source circuit, a selectable voltage gain circuit, and 
analog-to-digital converter, a read-only memory, a digital processor, and 
a display portion. 
While there has been illustrated and described what is at present 
considered to be a preferred embodiment of the present invention, it will 
be understood by those skilled in the art that various changes and 
modifications may be made, and equivalents may be substituted for elements 
thereof without departing from the true scope of the invention. In 
addition, many modifications may be made to adapt a particular situation 
or material to the teachings of the invention without departing from the 
central scope thereof. Therefore, it is intended that this invention not 
be limited to the particular embodiment disclosed as the best mode 
contemplated for carrying out the invention, but that the invention will 
include all embodiments falling within the scope of the appended claims.