two matching early 20th century vintage industrial mottled brown glazed finish "triple cup" heavy duty suspension type transmission line insulators. the "clark" insulator was likely hung from a cross arm on a tower or pole, supporting the line conductor. suspension insulators are still used for very high voltage systems when it is not practical or safe to use other types of insulators. they have an advantage in that one or more of the insulators in a string can be changed out without replacing the entire string. the first electrical systems to make use of insulators were telegraph lines; direct attachment of wires to wooden poles was found to give very poor results, especially during damp weather. the first glass insulators used in large quantities had an unthreaded pinhole. these pieces of glass were positioned on a tapered wooden pin, vertically extending upwards from the pole's crossarm (commonly only two insulators to a pole and maybe one on top of the pole itself). natural contraction and expansion of the wires tied to these "threadless insulators" resulted in insulators unseating from their pins, requiring manual reseating. amongst the first to produce ceramic insulators were companies in the united kingdom, with stiff and doulton using stoneware from the mid 1840s, joseph bourne (later renamed denby) producing them from around 1860 and bullers from 1868. utility patent number 48,906 was granted to louis a. cauvet on july 25, 1865 for a process to produce insulators with a threaded pinhole. to this day, pin-type insulators still have threaded pinholes. the matching oversized insulators were fabricated by the clark electric & manufacturing co., new york city, ny. priced for the pair. measures 14 1/4 x 17 1/2 inches.