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 ;-)
[Table of Contents]
People old and young
enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published
from May 1959 through December 1971.
As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
So, for those who still appreciate a good resistor selection nomograph, here is one from the April 1967
edition of Electronics World.
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?
See all the available
Electronics World articles.
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.