What kind of dog was used in the RCA victor phonograph advertisement
with the words
My Masters Voice?

As the owner of two delightful mixed-breed terriers, I am pleased to
say that "Nipper," the famous RCA dog, was part bull terrier and part
fox terrier.

The history of Nipper (and his companion, the pup "Chipper," who
appeared in later RCA ads) may be found here:

Linda's Lighthouse Page 
http://www.ais.org/~lsa/nipper.html

An excerpt from the page linked above:

"Nipper is perhaps the best-known and most loved advertising
trademark. In the United States, we know him as the 'RCA dog.' But he
started life in Bristol, England in 1884. Nipper was a mutt, part bull
terrier and a trace of fox terrier.

When his master died he became the pet of the Barraud brothers, Mark
and Francis. At Francis Barraud's photographic studio, Nipper would
listen attentively to the old phonograph. One day it occurred to
Barraud that the dog might be waiting to hear his master's voice. This
inspired him to paint the oil (1895) of Nipper and the gramophone,
which is titled appropriately His Master's Voice."

Here are two more references to Nipper as a pit bull:

"RCA used Nipper, a pit bull of unknown ancestry, to illustrate the
clarity of sound emulating from it's phonograph bye showing the pit
bull being fooled into thinking he was actually hearing his master's
voice and not a recording."

Nickes Pitbull Page
http://home7.swipnet.se/~w-76223/history.htm

"Nipper, the black-and-white pit bull of an English artist called
Barraud, was making American advertising history as the dog in RCA
Victor's His Master's Voice."

Positive Pitbull Awareness Society
http://www.itsapitty.com/history.html

Here's a vintage image of the "His Master's Voice" logo:

Ande Rooney Signs
http://www.anderooney.com/Graphics/RCADisk.gif

Search terms used:

rca
nipper
"his master's voice"
pit bull
terrier

I hope this information is helpful. If any of the links do not
function, or if anything I've said is unclear or incomplete, please
request clarification before rating my answer, and I'll be glad to
offer further assistance.

Best wishes,
pinkfreud

What a great job for such a small price!  Thank you,this is an excellent service.