IS-4 through IS-7 (1915 to 1928) Previous
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With the introduction of the I-14, the IS-4 and IS-5 meters were introduced and were smaller than the IS-2 and IS-3, although they were similar in appearance. The IS-4 had a cast iron cover while the IS-5 had a cover with glass windows.

When the 25Hz I-14 was reworked into the I-15, the 25Hz IS-4 and IS-5 meters became the IS-6 and IS-7, respectively.

 
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