It is a screw in fuse to an electrical box. Probably used in 1920-1930s type wireing in houses. You look into the glass window and can see if the wire tab is burned out.
IH is right; it is a screw in fuse. I remember we had those when I was a child back in the 1950's. Whooops, did I just tell my age? I was a young child then . .lol Breezie
Many times when those type fuses would "blow," people would put a penny in the slot if they didn't have a replacement. Big mistake! That would defeat the purpose of the fuse and the result would often be a house fire.
Scott
PS. I'm sure that you can still find this type of fuse system in alot of older houses.
The only time we ever had trouble with one was when we [ as kids ] would plug up the Creepy Crawlers , Wood burner set, TV, and Electric train in the same room ,
Oops I forgot the tree with the bubbling lights.
They are still used and probably still sold today. I dont buy them because I have boxes full. A quick upgrade is the TD time delay type or you can buy a fuse with a breaker button.
i used to work making them for 27 yrs. all types mainly for air craft and helicopters. if you see the ones that say mech. products, jackson,michigan. rounds and squares and some as large as zippos andcigarette packs. my son said he seen the ones i made in a lot of military applications. iam old too.