April 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Even in the age of computers and smartphone applets for everything,
some people still are not comfortable with mathematics, even something
as relatively simple as Ohm's Law. There are still a lot of electronics
tinkerers around who grew up in the pencil and paper era, where
slide rules ruled the day. Those same folks broke out in a rash
at the very sight of one of those devices ;-)
So, for those who still appreciate a good resistor selection
nomograph, here is one from the April 1967 edition of Electronics
Here's a challenge for you recent college grads that are scoffing
at the guys who are right now having a grandkid print out a copy
of the nomograph for them: What is the resistor color code number
associated with the color of the nomograph?
By Sylvester Salva
Resistance values and power dissipations
may be readily determined by the use of a straightedge.
For the man involved in the electronics field, it frequently
becomes necessary to determine the value of a resistor and wattage.
The nomogram below was designed to give the resistor value and its
wattage simply by placing a straightedge from the potential scale
to the proper current scale and reading the answer on the Resistor
Value Scale or the Wattage Scale.
It will be noted that
there are two current scales on the nomogram. They are listed as
Current (Milliamps) Scale No. 1 and Current (Milliamps) Scale No.2.
Scale No.1 is used only when determining the value of the resistor.
Scale No. 2 is used for determining the wattage of the resistor.
Following are two examples illustrating the use of the nomogram.
Example No.1: It is found that a cathode bias resistor is
burned out and no schematic is available. Total current of the tube
- rated at 60 mA. Grid bias is -18 volts. Determine the value, in
ohms, and wattage of the resistor.
Solution: Extend a straight
line from 18 on the Potential Scale to 60 on the Current Sca1e No.
1. Read the value of the resistor at the intersection of this line
with the Resistor Value Scale as 300 ohms. To determine the wattage,
extend another line from 18 on the Potential Scale to 60 on the
Current Scale No. 2. At the intersection of this line with the wattage
scale, read the answer as slightly over 1 watt. Since the value
is higher than 1, use the next higher value, namely 2 watts.
Example No.2: Determine the value, in ohms, and wattage of a
resistor under the following conditions: Voltage is measured at
117 volts, current at 10 mA.
Solution: Extend a straight
line from 117 on the Potential Scale to 10 on the Current Scale
No. 1. At the intersection of this line with the Resistor Value
Scale read the value as 11.7 kohms. To determine the wattage, extend
another line from 117 on the Potential Scale to 10 on the Current
Scale No.2. At the intersection of this line with the Resistor Wattage
Scale, read the answer as just over 1 watt. Assigning the next highest
value, the answer then becomes 2 watts.
Nomographs Available on RF Cafe:
Voltage and Power Level Nomograph
- Voltage, Current, Resistance,
and Power Nomograph
- Earth Curvature Nomograph
- Coil Design Nomograph
Coil Inductance Nomograph
- Antenna Gain Nomograph
Posted March 18, 2012