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Model Railroader 1937 July
CONTENTS
BERKELEY & LINCOLN RR complete centralized train control and automatic signaling installation
REBUILDING AN HO MOTOR how a mantua permag motor was trimmed to fit a mall firebox
A NEW INTERCHANGE IDEA good will train of HO gauge midget pacific RR will tour the country
TWO-RAIL SIMPLIFIED the track circuits are simply arranged for any combination of switches
BUILDING A LOCOMOTIVE part 7 finishing cab and firebox; making and fitting domes and stacks
DRIVE WHEEL MACHINING detailed method for carrying out this important fob in your own shop
YOUNGSTOWN UNION TERMINAL RR Ogauge road follows a local situation which permits variety of prototype
Water column
Useful Accuracy in Computation.
ENGINEERS have a great regard for what are called significant figures. These are the digits in any number which play a part in determining its exact value. You may say that all the digits in a number play a part in determining its exact value, but not always. For instance, if I measure a 40" piece with a rule that won't measure closer than one inch it does me no good to say that one-third of it is 131/3", because I can't be sure of the 1/3".
Model railroaders as a class are given to quibbling about small dimensions and could profit by adopting the engineers' attitude toward significant figures, that is, never to bother with accuracy of computations beyond the limit of accuracy of the data being used or the possibility of use.
For instance, a model rail building an 0 gauge switch may take the full size A. A. R. switch dimensions and scale them down to 1%4" scale. Naturally he gets some mighty small fractions, as the original switch starts with measurements to fractions of an inch. The typical model rail will carry the switch lead computations out to the nearest 64th of an inch. Now the lead of an 0 gauge switch runs around one foot or more, and the nearest it can be measured is to the 16th of an inch, so why bother to compute any closer? It's a waste of time.
The same way with the overall length of a car. This is a distance long enough so that the working limits of accuracy in construction are not less than Ms", so why compute any closer? Like the sophomore engineering student, you can multiply and divide out to 'steen decimal places if you like arithmetic, but why?
The slide rule is the greatest help there is to leaving off useless fractions and decimal places. The value of the slide rule in computation work is not only in the actual time saved in computing, but also in the way it encourages being satisfied with answers that are in line with the practical requirements. We think every model rail should have a slide rule, not only because it's the easiest way to do proportion, which is scaling down, but because it
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