Photograph from the International Slide Rule Museum
(Click on image for a larger view in a new window.)

I chose this slide rule as a special rule for two reasons;

• that is appears to have been a popular slide rule to use in the Texas Slide Rule Competition;

Every year until about 1980 a slide rule competition was held in the State of Texas (US). The contestants had to complete a series of calculations in 30 minutes. As the competition was timed there was a premium on speed but the marking system also required high accuracy. If the answer was accurate to three place of decimals, three points were awarded but if the answer was off by more than 2 places in the third significant figure no marks were awarded. Whilst other rules were allowed, and were used, the Pickett "Texas Speed Rule" was most popular. Ref: Ron Manley

• the unusual second K scale on the slide.

Click on this link for a sample of the Slide Rule Competition (197 kB PDF). As has been stated the marking of the paper was tough and as you can imagine, determining a result to a degree of accuracy from the K scale itself after a transferred value is a tough call^†.

Scales on the Texas Speed Rule
This rule has a fairly standard set of scales as far as slide rules go, apart from the extra K scale on the slider.

Obverse (front) side: K, A [B, K, CI, C] D, L

Reverse (back) side: [T, ST, S, C] D
There are also two markers on the top stator, possibly for alignment purposes.

All scales are printed in black including the CI scale on the front. The only gauge marks are π on the A, B, CI, D and D scales and a marker on CI, C and D for the number of degrees in one radian (≈57.3).

General:
As one knows, using the A and B scales, multiplication and division can be carried out as if they were the normal C and D scales although some accuracy may be lost in the reading of results. In conjunction with the C and D scales, products of squares are easily calculated. The same ease with calculations involving cubes is found with the extra K scale. The power of the extra K scale lies in the fact that values not not need to be transferred as the second K scale is synchronized with the C scale on the slider.

The following examples illustrate how the second K scale can be used. Bear in mind, that users may use different ways to obtain the same result.

Example 1:

When using any slide rule to perform calculations, it is not just a matter of using the numbers which appear in the problem. Generally some rearrangement will be required, especially when using the A, B and K scales.

Upon some minor simplification and rearrangement;

This will actually give the correct values to use on the A and B scales. Recall that the A and B are double-decade logarithmic scales. The following algorithm in SR-ALGOL gives the solution to this problem. Clearly, when it mentions that the K scale is to be moved, it is referring to the K scale on the slider. (To activate the decoder, simply move your mouse over the command and click, the algorithm will appear as a pop-up window.)

[1B->391A;|->25.1B;93.1K->|;|->[1C;[1C=>2.2D;2.2K->|;[1C=>1.69D

Estimation of this result gives that the value is between 1 and 10, so 1.69 is the approximate result.

 

Example 2:

Again some minor rearrangement:

A possible method of solution is given below in algorithm syntax. The operations are carried out on the A and B scales.

|->3.21A;πC->|;|->]1C;|=>32.5A;325K->|;]1B=>68.8

Estimation of this result shows that the value is divided by a further factor of 100, meaning that this result must be multiplied by 10^-4 giving 0.00688 as the final approximation.

Other ways are possible, although longer, but this is not really of any great concern as it is up to the user to decide the best way for them to carry out the operations. A further example is given below.

|->3.21D;]1CI->3.21;|->πCI;[1C->|;|->πCI;|=>3.25D;325K->|;|=>6.88C

The extra K scale might be very useful, but careful work needs to be taken in the intermediate approximations to make sure that the relevant powers of ten have been taken into account.

 

 

^† By transferred, we mean arriving at a result on one scale and then physically moving that result to another scale which is not synchronized with the current working scale.