This framework of wood and wire-netting (chicken wire the Americans call it) was devised as a means to prevent enemy stick grenades from lodging on the roof of the tank.
They were made in England but only enough appear to have been ready for C Company to fit them, they were never seen on D Company tanks. Later on, when A Company arrived in France some of their tanks were seen fitted with them as well. The framework was extended on some tanks to cover the tops of the sponsons.
These devices were never used again after 1916, grenades were hardly ever thrown onto the roof of a tank and to some extent the frame inhibited the use of the upper hatch, especially as a means of escape. However they are sometimes seen as a characteristic of the early tanks.
During trials conducted later in the war a cluster of five grenades, detonated on the roof of a Mark I tank did some damage and could have injured a crew member. But individual grenades did no real harm at all.
Another device for preventing damage was a perforated burster plate, 4mm thick, which was to be fitted to the roof of some tanks on extended bolts, held some distance away from the structural armour to detonate incoming projectiles. Twenty five sets were made by Metropolitan in Birmingham but they were never used.
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