During the course of the series, the Jefferson Nickel
was struck in two different compositions. The first composition was the standard alloy used since the introduction of the denomination in 1866, while the second was used during World War II in an effort to preserve nickel and copper.
The original composition consisted of 75% copper and 25% nickel. The weight of uncirculated pieces is 5 grams, or 77 grains, with a diameter of 21.2 mm and a plain edge. These specifications apply to coins struck from 1938 to 1942, and 1946 to present.
From October 1942 until the end of 1945, a composition consisting of 56% copper, 35% silver and 9% manganese was used. The weight and diameter of the coins remained the same. The mintmark location was moved to the reverse of the coin above Monticello, where it appeared in larger size. This brief run represented the first time the five cent denomination contained silver since 1873.
taken from: http://jeffersonnickel.org/