Scientific - Vintage Sinclair

It was advertised as offering logs, trig, and true scientific notation over a 200-decade range and Sinclair claimed that these were features only available on machines costing £100 or more (at the time of this claim in late 1974 the Scientific was £32.35 built and £19.95 as a kit).

Unusually, it used reverse polish notation (RPN) ie. the operator was entered after the second number and so no equals key was needed.
For example to calculate 2 times 2, it is entered as:
2
+ (adds the 2 to zero)
2
X
Also, all numbers over 10 had to be entered as exponents, so 50 is entered as 5E1.

A big attraction was that it was so compact for the features it offered and easily carried around in the pocket.
The downside was that it was a bit flimsily constructed, especially the on / off key which was prone to wear and staying on permanently.

Scientific Programmable

Released in 1974, the Scientific Programmable had a 24-step programming ability, which meant it was highly limited for many purposes. It also lacked functions for the natural logarithm and exponential function. Constants used in programs were required to be integers, and the programming was wasteful, with start and end quotes needed to use a constant in a program.

However, included with the calculator was a library of over 120 programs that performed common operations in mathematics, geometry, statistics, finance, physics, electronics, engineering, as well as fluid mechanics and materials science.