Safety razors Main article: Safety razor

Safety razorsMain article: Safety razor.

A modern double-edged safety razorThe first safety razor protected the skin from all but the very edge of the blade and was invented in the late 19th century by a Frenchman, Jean-Jacques Perret, who was inspired by the joiner's plane. In 1875 it was marketed by the Kampfe Brothers as "the best available shaving method on the market that won’t cut a user, like straight steel razors."

In 1901, the American inventor King Camp Gillette, with the assistance of William Nickerson, invented a safety razor with disposable blades. Gillette realized that a profit could be made by selling an inexpensive razor with disposable blades. This has been called the Razor and blades business model, or a "loss leader", and has become a very common practice for a wide variety of products.

There are also safety razors that are made of inexpensive materials that are meant to be wholly disposable. One such device was patented in the late 1950s by American entertainer and inventor Paul Winchell.

Electric razors

Rotary razor: The electric razor (also known as the electric dry shaver) has a rotating or oscillating blade. The electric razor usually does not require the use of shaving cream, soap, or water. The razor may be powered by a small DC motor, which is either powered by batteries or mains electricity. Many modern ones are powered using rechargeable batteries. Alternatively, an electro-mechanical oscillator driven by an AC-energized solenoid may be used. Some very early mechanical shavers had no electric motor and had to be powered by hand, for example by pulling a cord to drive a flywheel.

The first electric razor was patented in 1928 by the American manufacturer Col. Jacob Schick. The Remington Rand Corporation developed the electric razor further, first producing the Remington brand of razor in 1937. Another important inventor was Prof. Alexandre Horowitz, from Philips Laboratories in the Netherlands, who invented the very successful concept of the revolving (rotary) electric razor. It has a shaving head consisting of cutters that cut off the hair entering the head of the razor at skin level. The major manufacturers introduce new improvements to the hair cutting mechanism of their products each few years. Each manufacturer sells several different generations of cutting mechanism at the same time, and for each generation, several models with different features and accessories to reach various price points. The improvements to the cutting mechanisms tend to 'trickle-down' to lower priced models over time.

Early versions of electric razors were meant to be used on dry skin only. Many recent electric razors have been designed to allow for wet/dry use, which also allows them to be cleaned using running water or an included cleaning machine, reducing cleaning effort. Some patience is necessary when starting to use a razor of this type, as the skin usually takes some time to adjust to the way that the electric razor lifts and cuts the hairs. Moisturizers designed specifically for electric shaving are available.

Radio active Carbon 14 dating of objects

Radiocarbon dating (sometimes simply known as carbon dating) is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 (14C) to estimate the age of carbonaceous materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" (BP), "Present" being defined as 1950. Such raw ages can be calibrated to give calendar dates. One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. When plants fix atmospheric carbon dioxide (CO2) into organic material during photosynthesis they incorporate a quantity of 14C that approximately matches the level of this isotope in the atmosphere (a small difference occurs because of isotope fractionation, but this is corrected after laboratory analysis). After plants die or they are consumed by other organisms (for example, by humans or other animals) the 14C fraction of this organic material declines at a fixed exponential rate due to the radioactive decay of 14C. Comparing the remaining 14C fraction of a sample to that expected from atmospheric 14C allows the age of the sample to be estimated.

The technique of radiocarbon dating was developed by Willard Libby and his colleagues at the University of Chicago in 1949. Emilio Segrè asserted in his autobiography that Enrico Fermi suggested the concept to Libby in a seminar at Chicago that year. Libby estimated that the steady state radioactivity concentration of exchangeable carbon-14 would be about 14 disintegrations per minute (dpm) per gram. In 1960, he was awarded the Nobel Prize in chemistry for this work. He first demonstrated the accuracy of radiocarbon dating by accurately estimating the age of wood from an ancient Egyptian royal barge for which the age was known from historical documents.