Send to a Friend View / Add Comments  
That 25 Ohm Rule Revisited

In my last email titled "What a Great Week of Discovery" (, I provided a link to a paper titled "EARTH WIRES OR THE EARTH AS AN ELECTRIC COMPLETER." That paper indicated that the Count De Moncel made an observation some time before 1884, that the resistance of the earth under very favorable conditions was equivalent to about 7 miles of 8 AWG. If you do the math, this calculates to approximately 24.64 ohms, which is almost 25 ohms, interesting.

Mr. Frank L. Pope's, mentor to Thomas Edison (, great book "Modern Practice of the Electric Telegraph", published in 1881 (, describes in Section 189 Measure of Resistance, that one of the units of resistance measurement was a Varley, which was equal to 25 ohms (

Varley Unit: A unit of electrical resistance equal to the resistance of 1 mile of copper telegraph wire. The unit was named for C. F. Varley, an engineer working for the Telegraph Company in Great Britain who devised a method of locating faults in underground wires by comparing their resistance to that of good wires

Why was the Varley used instead of the Ohm as the unit of measurement? I don't know if I'll get that question answered, but if 1 Varley is equal to 1 mile of copper telegraph wire, this is a convient unit to work with. Instead of saying that the circuit had a resistance of 250 ohms, one could say it had a resistance of 10 Varley's. Naturally I'm just guessing.

Yes, but where did the 25 ohms rule for a single ground rod come from? I don't know yet, but I'm guessing that they might have simply used the smallest unit of resistance measurement as the basis of the minimum resistance of the ground for the telegraph lightning arrestors (

By the way, the voltage required for the teletype was about 60V (at each end) to produce the necessary current flow of 50 mA for the circuit to operate. Amazing, in the mid 1860's, these guys used 120VDC ( and to transmitte electrical signals across the USA as well as across the oceans (

In doing the math, it appears that the total circuit resistance would be about 80 Varleys'.

Circuit Resistance in Ohms = Circuit Voltage/0.050A
Circuit Resistance in Ohms = 60V x 2/0.050A
Circuit Resistance in Ohms = 120V*/0.50A
Circuit Resistance in Ohms = 2,000 Ohms (how convenient of a number)

*Is this why Edison used 120V for the circuit voltage for indoor wiring?

Circuit Resistance in Varleys' = Resistance in Ohms/25 Ohms
Circuit Resistance in Varleys' = 2,000 ohms/25 Ohms
Circuit Resistance in Varleys' = 80 Varleys'

If you're ever in San Francisco, visit the excellent displays of early telegraph instruments, and even telegraph wire, at the Wells Fargo History Museum at 420 Montgomery Street. I'll make it a point to visit this museum the next time I'm in Frisco.

Well as you can see, I'm having a blast learning about the great men and the amazing things they did before Edison left the telegraph industry.

Well I've been working (if you want to call it that) very late tonight, or is that very early today, and it's time I disconnected my brain.

Good Night or Good Morning, and especially God Bless,

Mike Holt
[ View More Newsletters ] View / Add Comments  

09. Basic Electrical Theory - Standard Library DVD
Do you know where electricity comes from? To be able to say yes to that question, you must understand a bit about the physics of matter. What value does a brief study of the nature of matter have for the student of electrical theory? The understanding that comes from that study lays the foundation for understanding electrical theory. Only when you know the theory can you truly have confidence in the practical aspects of your electrical work.

This all new 430-plus page full-color textbook provides hundreds of illustrated graphics, detailed examples, practice questions, practice exams, summaries and a comprehensive practice final exam. Includes textbook/workbook and three DVDs; Electrical Fundamentals and Basic Electricity; Electrical Circuits, Systems and Protection; and Alternating Current, Motors, Generators, and Transformers. Purchased separately this Library would cost $377, you save $102.

Product Code: ETLIBD
Pages: 352
Illustrations: 278
MultiMedia: DVD

DVD/CD Return Policy: Unopened sealed DVD's or CD's may be returned within 10 days and we will credit your credit card or issue a refund for the price of the item(s). Opened items may not be returned unless they arrive defective.

Table of Contents
Sample Pages
Sample Graphic

  Send to a Friend View / Add Comments  

  [ Back to Top ]

Copyright © 2005 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)