A “soda jerk” (or “soda jerker”) is a person employed behind a soda fountain, “jerking”  the fountain spigot to make a soda or an ice cream soda or other such drinks. “Soda water jerkers and ice cream dishers smile at the sweltering crowds” was printed in the Daily Telegraph (Harrisburg, PA) on July 15, 1871. “A sensitive soda jerker” was printed in the book The Groceryman and Peck’s Bad Boy (1883) by George Wilbur Peck.
 
“Soda jerk” was printed in the Tampa (FL) Morning Tribune on January 1, 1908. “Soda jerks” was printed in the Evening Herald (Huntington, IN) on July 17, 1911. By about 1920, “soda jerk” would be used more often than “soda jerker.” By the 1960s, however, the soda fountains mostly disappeared, replaced by fast food restaurants and supermarkets selling such drinks.
 
Formally, an early term for “soda jerk(er)” was “soda clerk.” “WANTED—Soda Water Clerk—One acquainted with the business, and accustomed to the preparing of Syrups” was printed in the Daily Picayune (New Orleans, LA) on May 26, 1859.
 
A fancier name for a soda jerk(er) is a “sweet water chemist.”
   
       
Wikipedia: Soda jerk
A soda jerk (or soda jerker) is a person—typically a youth—who operates the soda fountain in a drugstore, often for the purpose of preparing and serving soda drinks and ice cream sodas. This was made by putting flavored syrup into a specially designed tall glass and adding carbonated water. One or two scoops of ice cream, or occasionally malt powder, could be added. The result was served with a long-handled spoon, most commonly known as a “soda spoon”, and drinking straws.
 
Origin of term
The term ‘soda jerk’ was a pun on ‘soda clerk’, the formal job title of the drugstore assistants who operated soda fountains. It was inspired by the “jerking” action the server would use to swing the soda fountain handle back and forth when adding the soda water. The soda fountain spigot itself typically was a sturdy, shiny fixture on the end of a pipe or other similar structure protruding above the counter, curving towards where the glasses would be filled. All of the drinks were made of unflavored carbonated water. Consequently, the tap handle was large, as the soda jerker would use it frequently. This made the mixing of drinks a center of activity at the soda fountain.
 
(Oxford English Dictionary)
soda-jerk  n.
1922   Collier’s 17 June 4/1   You can tell a big-league head soda jerk by the way he picks up a glass, but the acid test is what kind of chocolate sirup he can make.
 
soda-jerker  n.  [jerk v.1 7] one who mixes and sells soft drinks, etc., at a soda-fountain.
1883   G. W. Peck Groceryman & Peck’s Bad Boy 137   A sensitive soda jerker..feels that it is worse than three card monte.
1932   P. G. Wodehouse Louder & Funnier 48   He..is now a soda-jerker in a small town in Kansas.
         
26 May 1859, Daily Picayune (New Orleans, LA), “Wants,” pg. 1, col. 4:
WANTED—Soda Water Clerk—One acquainted with the business, and accustomed to the preparing of Syrups. Apply to JAMES SYME, 134 Canal st.
 
15 July 1871, Daily Telegraph (Harrisburg, PA), pg. 3, col. 1:
SODA WATER jerkers and ice cream dishers smile at the sweltering crowds.
 
Google Books
Peck’s Bad Boy, No. 2:
The Groceryman and Peck’s Bad Boy:
Being a Continuation of Peck’s Bad Boy and His Pa
By George Wilbur Peck
Chicago, IL: Belford, Clarke & Co.
1883
Pg. 137:
He feels that he is a fraud, and when he puts a little syrup in a tumbler, and fires a little sweetened wind and water in it until the soap suds fills the tumbler, and charges ten cents for that which only costs a cent, a sensitive soda jerker, who has reformed, feels that it is worse than three-card monte.
     
10 October 1888, The Morning Star (Rockford, IL), pg. 4, col. 7:
Mr. Stevens, the soda water jerker at Porter’s drug store, does not have a great deal to do this kind of weather, and he went out driving last evening.
 
20 June 1890, Chicago (IL) Tribune, “The Man at the Fountain,” pg. 4, col. 7:
In winter time most soda-water jerkers work in the oyster houses. Soda-water jerkers, as a rule, are honest.”
   
1 January 1908, Tampa (FL) Morning Tribune, “Bond Ordinance May Be Delayed,” pg. 10, col. 4:
“Why, when I went to a cool drink stand for a drink to-day,” echoed Councilman Blackmon, “the soda jerk actually asked me what kind of milk I wanted, as if he carried various kinds—good, bad and indifferent.”
 
17 July 1911, Evening Herald (Huntington, IN), pg. 5, col. 4:
SLOGAN SODA IS THE LATEST SOFT DRINK
“Huntington—Opportunity’s Gateway,” is the city slogan and also of a new drink now served by many of the soda jerks about the city.
       
24 August 1911, Wagoner County Record

Grady Turner, the popular “soda jerk” at the Wagoner Drug Co., spent the past week with home folks at Stigler.
 
Google Books
6 May 1915, The N.A.R.D. (National Association of Retail Druggists) Journal (Chicago, IL), “Ita Dicere” by Finney Briggs, pg. 205, col. 2:
So, Druggist Friends,
It’s up to you :
See what your “soda-jerk”
Can do to plan, invent,
And eke, devise some grape juice drinks
 
OCLC WorldCat record
Heard on Main Street : the book of the bank clerk and the soda jerker
Author: W O Saunders
Publisher: Elizabeth City, N.C. : The Independent, 1927.
Edition/Format:   Print book : English : [Author’s ed.]
 
OCLC WorldCat record
Linguistic Concoctions of the Soda Jerker
Author: Harold W Bentley
Edition/Format: Article Article : English
Publication: American Speech, v11 n1 (19360201): 37-45
 
OCLC WorldCat record
Hash house lingo : the slang of soda jerks, short-order cooks, bartenders, waitresses, carhops and other denizens of yesterday’s roadside
Author: Jack Smiley; Paul Dickson
Publisher: Mineola, New York : Dover Publications, Inc., [2012]
Edition/Format:   Print book : English